[llvm] [CI] Disable flang from pre-commit tests when Flang files are not touched (PR #93485)

[Renato Golin via llvm-commits]

rengolin wrote:

> This issue did take some time to fix because it was not reproducible on any of the Windows machines we tried. One of the past issues that was brought up (#77086) was a bot issue as pointed out by @joker-eph in [#77086 (comment)](https://github.com/llvm/llvm-project/issues/77086#issuecomment-1878905702). I don't remember what the third issue was.

I can't remember either, tbh. This was last year, and IIRC on Linux.

Right now I don't mind if we just wait until it crashes again, but we need to be more responsive. If we get instability and it's hard to reproduce, then the tests must be taken down from pre-commit CI until it can be reproduced, fixed and shown stable.

This is the kind of cost that is non-trivial and it makes no sense for everyone to have to pay when only a few people can actually do anything about it.

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