Auto merge of #145874 - Kobzol:fix-dist-builds, r=jieyouxu

Remove unnecessary stage2 host builds from cross-compiled dist builders

This is a repeated regression (rust-lang/rust#138004, rust-lang/rust#138123) that was reintroduced in rust-lang/rust#145472. I thought that we have a test for it, but alas, the "correct" test required `--disable-docs`. I added the test in this PR, and re-added the `dist::Std` build optimization that solves this.

r? `@jieyouxu`

Author: bors