Auto merge of #116532 - onur-ozkan:enable-rustflags-bootstrap-on-bootstrap, r=albertlarsan68

Add RUSTFLAGS_BOOTSTRAP to RUSTFLAGS for bootstrap compilation

Adds `RUSTFLAGS_BOOTSTRAP` to `RUSTFLAGS` for bootstrap compilation when `RUSTFLAGS_BOOTSTRAP` exists in the environment. With this PR, `RUSTFLAGS_BOOTSTRAP` will affect every build(as we already do for rustc and std) compiled with stage0 compiler.

Resolves #94234

Author: bors