ci: move mirrors to their standalone bucket

Currently mirrors are stored in the rust-lang-ci2 S3 bucket along with
CI toolchains. This is problematic for multiple reasons:

- CI IAM credentials are allowed to both edit and delete those files.
  A malicious user gaining access to those credentials would be able to
  change our mirrored dependencies, possibly backdooring the compiler.

- Contents of the rust-lang-ci2 bucket are disposable except for the
  mirrors' content. When we implement backups for S3 buckets we'd have
  to replicate just that part of the bucket, complicating the backup
  logic and increasing the chance of mistakes. A standalone bucket will
  be way easier to backup.

This commit switches our CI to use the new rust-lang-ci-mirrors bucket.

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