Optimise rspec:coverage

Downloading rails-cache takes 2 minutes out of a 7 minute job: https://gitlab.com/gitlab-org/gitlab/-/jobs/730262074#L34

Investigate if it is possible to use reduced bundle, or install gems directly and cache specifically for this job, to run the scripts in rspec:coverage, in order to avoid depending on rails-cache.

Related #246884 (comment 410279616)