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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

This also does nothing to adjust the behavior on the buildbots. That needs to get in https://github.com/llvm/llvm-zorg.

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