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

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

> > I agree with @joker-eph we should still build and test it when flang/ files are touched.
> 
> Agree. What's the right incantation to make that work in our CI?

You'd need to remove `flang` from `compute-projects-to-test`.

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