Add guard to `replace_symlink` on Windows (#4519)
`junction::create` apparently will happily succeed but not create a link
to files? Since our symlink function does not indicate that it cannot
handle files, this was quite surprising.
Tested over in #4509 which previously failed on an assertion that
`black.exe` existed.
```
error: Failed to install entrypoint
Caused by: Cannot create a junction for [TEMP_DIR]/tools/black/Scripts/black.exe: is not a directory
```
We should file an issue upstream too, I think?
