Rollup merge of #146435 - Kobzol:gcc-download-default-true, r=GuillaumeGomez

Change the default value of `gcc.download-ci-gcc` to `true`

It makes sense for the vast majority of uses (rust-lang/rustc-dev-guide#2587 (comment)).

r? ```@GuillaumeGomez```

Author: Zalathar