Python: introduce NIX_PYTHONEXECUTABLE in order to set sys.executable

This is needed in case of `python.buildEnv` to make sure sys.executable
does not point to the unwrapped executable.

pkgs/development/interpreters/python/sitecustomize.py

@@ -8,11 +8,19 @@ The paths listed in `PYTHONPATH` are added to `sys.path` afterwards, but they
 will be added before the entries we add here and thus take precedence.
 
 Note the `NIX_PYTHONPATH` environment variable is unset in order to prevent leakage.
+
+Similarly, this module listens to the environment variable `NIX_PYTHONEXECUTABLE`
+and sets `sys.executable` to its value.
 """
 import site
+import sys
 import os
 import functools
 
 paths = os.environ.pop('NIX_PYTHONPATH', None)
 if paths:
     functools.reduce(lambda k, p: site.addsitedir(p, k), paths.split(':'), site._init_pathinfo())
+
+executable = os.environ.pop('NIX_PYTHONEXECUTABLE', None)
+if 'PYTHONEXECUTABLE' not in os.environ and executable:
+    sys.executable = executable