Rollup merge of #129651 - onur-ozkan:stage0-target-sanity-check, r=Kobzol

skip stage 0 target check if `BOOTSTRAP_SKIP_TARGET_SANITY` is set

When adding a new target to `rustc` and extending `STAGE0_MISSING_TARGETS`, there is a chance that in the merge CI bootstrap target sanity check might fail [here](https://github.com/rust-lang/rust/blob/26d27b7c8729fb61fe8321fcd2ce734a79aa695d/src/bootstrap/src/core/sanity.rs#L243-L256) because the stage 0 compiler will assume to already support the new target since `opt-dist` uses the previously compiled compiler as the stage 0 compiler.

This PR skips this check if `BOOTSTRAP_SKIP_TARGET_SANITY` is set, and makes `opt-dist` to set `BOOTSTRAP_SKIP_TARGET_SANITY` so bootstrap doesn't run this logic for opt-dist tests.

Fixes rust-lang/rust#127021 (comment).

Zulip thread: https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/.60STAGE0_MISSING_TARGETS.60.20seems.20to.20check.20stage1

Blocker for rust-lang/rust#127021

Author: matthiaskrgr