[llvm] [CI] Disable precompiled headers in monolithic-*.sh (PR #143369)

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

> I don't know whether these are the right scripts, but the concept looks good. Thanks!

These are the ones that run in GitHub actions, e.g. in the checks running for this PR. I've not touched anything CI-related in LLVM before so there might be something else that should be adjusted too.

https://github.com/llvm/llvm-project/pull/143369