Rollup merge of #42695 - Mark-Simulacrum:fix-verbose, r=alexcrichton

Use custom cargo/rustc paths when parsing flags.

Fixes https://github.com/rust-lang/rust/issues/41779, probably also https://github.com/rust-lang/rust/issues/42543 (I think they're duplicates).

I'm not entirely happy with the implementation, since it means we parse the configuration twice, but it's the minimal solution. I think the other choice is to move both calls to Config::parse inside Flags::parse and merge them, but I don't know if that's a good idea.

r? @alexcrichton