Auto merge of #116983 - Urgau:prepare-bootstrap-for-new-check-cfg, r=Kobzol

Prepare the `bootstrap` tool for the new check-cfg syntax

This PR prepare the `bootstrap` tool for the [new check-cfg syntax](rust-lang/rust#111072) as well as the according [changes to Cargo](rust-lang/cargo#12845).

~~Note that while the new syntax can technically available on stage > 2, we actually cannot use it since we need a cargo version that supports the new syntax which won't happen until the next beta bump (if I understand everything correctly).~~

r? bootstrap

Author: bors