Rollup merge of #121091 - onur-ozkan:bypass-stage0-download-in-tests, r=albertlarsan68

use build.rustc config and skip-stage0-validation flag

This change helps us to bypass downloading the beta compiler in bootstrap tests.

more context: https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/tests.20causing.20downloads.20of.20native.20rustc.20for.20other.20platforms/near/421467975

Author: matthiaskrgr