Rollup merge of #145291 - Zalathar:no-warning, r=Kobzol

bootstrap: Only warn about `rust.debug-assertions` if downloading rustc

The changes in rust-lang/rust#145149 had the unwanted side-effect of causing bootstrap to *always* warn about `rust.debug-assertions = true`, even if rustc isn't going to be downloaded anyway.

cc ``@Shourya742`` ``@Kobzol``