Nix: Fix overrides so that they work with the current nixpkgs unstable

pkgs/python-packages-overrides.nix

@@ -190,7 +190,8 b' self: super: {'
       pkgs.openldap
       pkgs.openssl
     ];
-    NIX_CFLAGS_COMPILE = "-I${pkgs.cyrus_sasl}/include/sasl";
+    # TODO: johbo: Remove the "or" once we drop 16.03 support.
+    NIX_CFLAGS_COMPILE = "-I${pkgs.cyrus_sasl.dev or pkgs.cyrus_sasl}/include/sasl";
   });
 
   python-pam = super.python-pam.override (attrs: