Rollup merge of #120096 - onur-ozkan:rustc_bootstrap, r=dtolnay

Set RUSTC_BOOTSTRAP=1 consistently

Fixes https://internals.rust-lang.org/t/rust-compiler-with-parallel-build/20099 which is a regression from #120001

cc `@dtolnay` `@petrochenkov`

Author: matthiaskrgr