Rollup merge of #142692 - Kobzol:bootstrap-small-check-cleanup, r=jieyouxu

Assorted bootstrap cleanups (step 3)

I keep failing to unwrap the gordic knot of the logic of checking tools in bootstrap 😖 So in the meantime I at least want to upstream some cleanups I did along the way.

Since some time ago, we have separate steps for Clippy, so it shouldn't ever happen again that the check steps would be invoked with `builder.kind == Clippy`.

r? `@jieyouxu`

Author: tgross35