Auto merge of #1486 - pietroalbini:switch-mirrors-bucket, r=gnzlbg

ci: switch to the rust-lang-ci-mirrors bucket for mirrors

Previously mirrors were stored in the rust-lang-ci2 bucket, which is
meant to store temporary data (the CI artifacts). This switches the code
to fetch from the new mirrors bucket.

The old files won't be removed, but they won't be backed up either.

r? @gnzlbg