Rollup merge of #144056 - Kobzol:gcc-build-src, r=nikic

Copy GCC sources into the build directory even outside CI

It takes ~3.5s on my Linux notebook to perform the copy, but it should only be executed when we actually go build GCC, and that will almost certainly take much longer :) So I think it should be fine. At least we won't be polluting the source directory for local builds.

Fixes: https://github.com/rust-lang/rust/issues/143986

r? `````@nikic`````