Auto merge of #117813 - onur-ozkan:simplify-download-ci-llvm-option, r=Mark-Simulacrum

deprecate `if-available` value of `download-ci-llvm`

This PR deprecates the use of the `if-available` value for `download-ci-llvm` since `if-unchanged` serves the same purpose when no changes are detected. In cases where changes are present, it is assumed that compiling LLVM is acceptable (otherwise, why make changes there?).

This was probably missing in the #110087 issue before.

cc `@RalfJung`

Author: bors