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

[David Spickett via llvm-commits]

DavidSpickett wrote:

I agree this is a useful change to make. Wait for @boomanaiden154 to review and confirm this is the right place to make the change.

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