Optimize the rspec:coverage and rspec:undercoverage jobs for speed
## Context
* `rspec:coverage` and `rspec:undercoverage` are the last jobs we run on the critical path of code pipelines. As soon as they finish, the pipeline is finished.
* https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428375072
* https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428375524
## Goal
Investigate and experiment to see whether we could make those jobs faster.
## Ideas
### `rspec:coverage`
* [Do we need a **git clone**?](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428582192)
* [Do we need to install gems, or do we just need `simplecov`?](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428583360)
* [Could we just use an **alpine ruby image**?](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428583360)
### `rspec:undercoverage`
* [Could we just use an alpine ruby image?](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428594580)
* [Could we just install the gems we need? (e.g. `undercover` and dependencies from `Gemfile.lock`)](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428595113)
issue