RUNFILES_MANIFEST_FILE env var should be allowed to be unset on Windows when --experimental_enable_runfiles is set

[emmaworley]

Currently these lines will make windows builds fail when --experimental_enable_runfiles (or --enable_runfiles in the 5.0 RC) are set, as they unset the RUNFILES_MANIFEST_FILE environment variable. Is this check even necessary in the first place?

[jesseschalken]

Just ran into this. I have to turn on --enable_runfiles on Windows to get some other rules to work, but it breaks the Kotlin part of my build due to this issue. 😢