[llvm] [CI] Disable flang from Windows pre-commit tests (PR #93485)

[Renato Golin via llvm-commits]

rengolin wrote:

> It will be great if you can write a new post in discourse before going ahead. The original issue regarding the previous post was fixed, the last patch in the sequence was #93295. The other issue discussed in the post was OOM. Do you still see the problems after #93329?

This has nothing to do with the importance of Flang or Windows, only that we should not run tests when we don't touch those files. No need for an RFC here, this is just basic good CI practices.

The fact that those tests are unstable for over a year, for different issues, on Linux and Windows is just the trigger to do this in the first place, not the justification to do so.

https://github.com/llvm/llvm-project/pull/93485