Fix environment not set correctly for Windows subprocess backend

[ethanbb]

This is a follow-up on #327; not sure exactly why it didn't come up then, but maybe because I now have a UNC path as my working directory; cmd rejects this and defaults to C:\Windows. But the working directory shouldn't matter anyway.

I was getting this error:

Fatal Python error: _Py_HashRandomization_Init: failed to get random numbers to initialize Python

which apparently can happen when trying to invoke python using Popen if the environment isn't set correctly: https://stackoverflow.com/a/72845540

I realized that I was adding semicolons to the end of each SET line in the .bat file, which I believe was setting the variables to incorrect values. Deleting the semicolon fixed this for me.

[ethanbb]

Still getting segfault for this on the Windows run which I think is due to tensorflow...I think this is safe to merge though, if the semicolon is needed for anything it's by accident.