pyoxidizer: re-install PYTHONPATH behavior

Hooks and extensions can rely on PYTHONPATH value. Yet the pyoxidized binary
ignore the variable. So we manually reinstall the behavior to fix various tests
and restore the expected behavior.

This positively affects:
- test-extension.t
- test-hook.t

Differential Revision: https://phab.mercurial-scm.org/D11302

rust/hgcli/pyoxidizer.bzl

@@ -36,6 +36,14 b' IS_WINDOWS = "windows" in BUILD_TARGET_T'
 
 # Code to run in Python interpreter.
 RUN_CODE = """
+import os
+import sys
+extra_path = os.environ.get('PYTHONPATH')
+if extra_path is not None:
+    # extensions and hooks expect a working python environment
+    # We do not prepend the values because the Mercurial library wants to be in
+    # the front of the sys.path to avoid picking up other installations.
+    sys.path.extend(extra_path.split(os.pathsep))
 import hgdemandimport;
 hgdemandimport.enable();
 from mercurial import dispatch;