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

[Renato Golin via llvm-commits]

rengolin wrote:

> Even with pre-commit enabled it happened a couple of times that some patches were committed even with obvious failure in the pre commit CI.

This is what I'm trying to avoid. We can't really move to better CI if we keep asking people to "just ignore the failures if you don't understand".

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