handle restoration of windows environment variables correctly

[bhcleek]

Fixes #2286

[bhcleek]

@glebtv @aidanbrumsickle can you confirm that this works for you?

[aidanbrumsickle]

Looks good.

Deleted a few Go tool binaries and ran :GoInstallBinaries and it worked. I then did :GoBuild to make sure the go executable was still available, which it was, and I did :echo $PATH afterward and it was fine. Running :GoUpdateBinaries now since I believe that calls the same function.

[bhcleek]

Thank you for the assistance!

[aidanbrumsickle]

Thanks for fixing it so quickly! I use this plugin all the time and it's been getting better and better.

[bhcleek]

Thank you for the accolades. I'm trying hard to maintain high quality and resolve existing issues before adding anything new.