use precompiled rustc for non-dist builders Makes non-dist builders to use precompiled CI rustc by default if they are available for the target triple. As we are going to make `rust.download-rustc=if-unchanged` default option with https://github.com/rust-lang/rust/pull/119899, we need to make sure `if-unchanged` logic never breaks and works as expected. As an addition, this will significantly improve the build times on CI when there's no change on the compiler. blocker for #119899 try-job: x86_64-gnu-nopt try-job: aarch64-apple