bootstrap python doesn't work with symbolic links on windows remote execution.

This is most likely a different exposure of the same underlying issue where symlinks aren't propagating properly for windows re. In this case, the actual python:
import "module", isn't finding a module that is supposedly available via symlinks:

```
Action failed: root//Test/Core:Core (cxx_compile src/Test.cpp)
Remote command returned non-zero exit code 1
stdout:
stderr:
Traceback (most recent call last):
File "C:\worker\work\3\0\exec\buck-out\v2\gen\prelude\749e8add6e5f1364\cxx\tools\__dep_file_processor__\dep_file_processor.py", line 10, in
from makefile_to_dep_file import process_dep_file
ModuleNotFoundError: No module named 'makefile_to_dep_file'
```
Changing this: https://github.com/facebook/buck2/blob/51d026458080a36d8342323313f4c0aa53302abc/prelude/python_bootstrap/python_bootstrap.bzl#L41
to use `copied_dir` bypasses the issue.

Related to #673 and #674