Fix path normalization on Windows

[Richienb]

os.path.abspath uses forward slashes on posix and back slashes on Windows.
This is a problem because the next line of code splits the string by (only) forward slashes.
Instead, we can use posixpath.normpath which always uses forward slashes.

[Lucaweihs]

Hi @Richienb, in principle I am happy to make a change like this, though I think we'd need to do something like:

rel_base_dir = posixpath.normpath(os.path.relpath(os.path.abspath(args.experiment_base), os.getcwd()))

to retain the same behavior of ensuring the path is relative the current working directory. Before making this change though: this change suggests that you're trying to get allenact working with Windows. Have you successfully done so? This isn't a use case we officially support (as most development is done on Mac/Linux) but I'd be interested in hearing otherwise.

[Richienb]

I have not yet succeeding in running allenact on Windows, this pr contains a change I made which fixes an error that was thrown, though there are likely more.