Auto merge of #122709 - onur-ozkan:use-precompiled-rustc-by-default, r=Mark-Simulacrum

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 rust-lang/rust#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

Author: bors