nix: Use --set instead of --prefix for setting the PATH / PYTHONPATH

Using --set will not append the system PATH / PYTHONPATH to the
environment variable during wraping of executables. This will "lock-out"
programs and python modules from the system and only allow the ones
which are set during wrapping.

This means e.g. of git we make only our custom git available and
if thats missing we get an error instead of silently using the system
git (if installed)

default.nix

@@ -1,153 +1,153 b''
 # Nix environment for the community edition
 #
 # This shall be as lean as possible, just producing the rhodecode-vcsserver
 # derivation. For advanced tweaks to pimp up the development environment we use
 # "shell.nix" so that it does not have to clutter this file.
 
 { pkgs ? (import <nixpkgs> {})
 , pythonPackages ? "python27Packages"
 , pythonExternalOverrides ? self: super: {}
 , doCheck ? true
 }:
 
 let pkgs_ = pkgs; in
 
 let
   pkgs = pkgs_.overridePackages (self: super: {
     # Override subversion derivation to
     #  - activate python bindings
     subversion = let
       subversionWithPython = super.subversion.override {
         httpSupport = true;
         pythonBindings = true;
         python = self.python27Packages.python;
       };
     in pkgs.lib.overrideDerivation subversionWithPython (oldAttrs: {
       patches = (oldAttrs.patches or []) ++
         pkgs.lib.optionals pkgs.stdenv.isDarwin [
           # johbo: "import svn.client" fails on darwin currently.
           ./pkgs/subversion-1.9.4-darwin.patch
         ];
     });
   });
 
   inherit (pkgs.lib) fix extends;
 
   basePythonPackages = with builtins; if isAttrs pythonPackages
     then pythonPackages
     else getAttr pythonPackages pkgs;
 
   elem = builtins.elem;
   basename = path: with pkgs.lib; last (splitString "/" path);
   startsWith = prefix: full: let
     actualPrefix = builtins.substring 0 (builtins.stringLength prefix) full;
   in actualPrefix == prefix;
 
   src-filter = path: type: with pkgs.lib;
     let
       ext = last (splitString "." path);
     in
       !elem (basename path) [
         ".git" ".hg" "__pycache__" ".eggs" "node_modules"
         "build" "data" "tmp"] &&
       !elem ext ["egg-info" "pyc"] &&
       !startsWith "result" path;
 
   rhodecode-vcsserver-src = builtins.filterSource src-filter ./.;
 
   pythonGeneratedPackages = self: basePythonPackages.override (a: {
     inherit self;
   })
   // (scopedImport {
     self = self;
     super = basePythonPackages;
     inherit pkgs;
     inherit (pkgs) fetchurl fetchgit;
   } ./pkgs/python-packages.nix);
 
   pythonOverrides = import ./pkgs/python-packages-overrides.nix {
     inherit
       basePythonPackages
       pkgs;
   };
 
   version = builtins.readFile ./vcsserver/VERSION;
 
   pythonLocalOverrides = self: super: {
     rhodecode-vcsserver = super.rhodecode-vcsserver.override (attrs: {
       inherit
         doCheck
         version;
       name = "rhodecode-vcsserver-${version}";
       releaseName = "RhodeCodeVCSServer-${version}";
       src = rhodecode-vcsserver-src;
 
       propagatedBuildInputs = attrs.propagatedBuildInputs ++ ([
         pkgs.git
         pkgs.subversion
       ]);
 
       # TODO: johbo: Make a nicer way to expose the parts. Maybe
       # pkgs/default.nix?
       passthru = {
         pythonPackages = self;
       };
 
       # Somewhat snappier setup of the development environment
       # TODO: move into shell.nix
       # TODO: think of supporting a stable path again, so that multiple shells
       #       can share it.
       shellHook = ''
         # Set locale
         export LC_ALL="en_US.UTF-8"
 
         tmp_path=$(mktemp -d)
         export PATH="$tmp_path/bin:$PATH"
         export PYTHONPATH="$tmp_path/${self.python.sitePackages}:$PYTHONPATH"
         mkdir -p $tmp_path/${self.python.sitePackages}
         python setup.py develop --prefix $tmp_path --allow-hosts ""
       '';
 
       # Add VCSServer bin directory to path so that tests can find 'vcsserver'.
       preCheck = ''
         export PATH="$out/bin:$PATH"
       '';
 
       postInstall = ''
         echo "Writing meta information for rccontrol to nix-support/rccontrol"
         mkdir -p $out/nix-support/rccontrol
         cp -v vcsserver/VERSION $out/nix-support/rccontrol/version
         echo "DONE: Meta information for rccontrol written"
 
         ln -s ${self.pyramid}/bin/* $out/bin/
         ln -s ${self.gunicorn}/bin/gunicorn $out/bin/
 
         # Symlink version control utilities
         #
         # We ensure that always the correct version is available as a symlink.
         # So that users calling them via the profile path will always use the
         # correct version.
         ln -s ${pkgs.git}/bin/git $out/bin
         ln -s ${self.mercurial}/bin/hg $out/bin
         ln -s ${pkgs.subversion}/bin/svn* $out/bin
 
         for file in $out/bin/*; do
           wrapProgram $file \
-            --prefix PATH : $PATH \
-            --prefix PYTHONPATH : $PYTHONPATH \
+            --set PATH $PATH \
+            --set PYTHONPATH $PYTHONPATH \
             --set PYTHONHASHSEED random
         done
       '';
 
     });
   };
 
   # Apply all overrides and fix the final package set
   myPythonPackages =
     (fix
     (extends pythonExternalOverrides
     (extends pythonLocalOverrides
     (extends pythonOverrides
              pythonGeneratedPackages))));
 
 in myPythonPackages.rhodecode-vcsserver