Rollup merge of #142624 - Kobzol:bootstrap-fix-host, r=jieyouxu

Actually take `--build` into account in bootstrap

I went back 20 *stable* versions of Rust and I couldn't find this flag actually being used. Despite some of our CI workflows actually set this flag (!).

I added destructuring of the flags to make sure that this doesn't happen again. It found one more duplicated CLI flag.

r? ```@jieyouxu```

Author: Kobzol