Fix site customize 2

[airportyh]

Why

Oops, I merged the #272 before testing. sitecustomize.py broke: it contained the path to the file rather than the contents. This fixes it (version 2).

Test Plan

1. create a Python Repl
2. pip install gunicorn
3. cat .pythonlibs/bin/gunicorn see that the shebang line starts with #!/usr/bin/env python3
4. make sure template tests (in particular pip and poetry tests) pass
5. pip install --upgrade pip
6. which pip to see we are using the pip inside .pythonlibs
7. try using that pip to install a thing: pip install flask

[airportyh]

Upgradable pip!

[airportyh]

Verified the shebang line works even with the upgraded version of pip.