Fix some tooling for windows
# What does this PR do?
This PR fixes a small issue on the `check_repo.py` script with the encodings. I have also updated the `Makefile` to move the creation of the variables `fork_point_sha`, `joined_dirs` and `modified_py_files` in the `modified_only_fixup` target because they are used only there.

The makefile raises an error on Windows environment when creating the `modified_py_files` variable:
```
'tests' is not recognized as an internal or external command, operable program or batch file
```

ping @stas00 and @sgugger 