Rollup merge of #145654 - Kobzol:gcc-ci-root, r=jieyouxu

Download CI GCC into the correct directory

While doing various experiments with stage3 cross-compilations, I realized that bootstrap is unable to download LLVM from CI for a non-host target, which is quite annoying. Fixing this for LLVM will take some work, but in the meantime we can easily fix this for `download-ci-gcc`, which was implemented in a much more self-contained way.

Author: jhpratt