Switch `ruleset` to use persistent cache mount
The ruleset switches to do zero GitLab CI caching.
It is expected that the disk/repository cache will be stored in a persistent mount between runs.
This can vastly speed up Bazel execution.
Bazel 8.3+ can use --repo_contents_cache to concurrently share the cached contents.
As of this merge request our persistent-mount runners have a mount at /mnt/cache. We set CI_PERSISTENT_CACHE_DIR to /mnt/cache in GitLab.
Edited by Matthew Clarkson