Make coverage CI jobs use BUNDLE_ONLY and not rely on main codebase

Context

Related to #416630 (closed).

First iteration of #416630 (comment 1474093453).

This iteration paves the way for a few future improvements for the rspec:coverage job:

1. Use a smaller, dedicated coverage gems cache, to only use the gems we need for Simplecov to function (downloading the cache takes ~15s)
2. Do not use git clone to fetch the codebase: fetch only the minimum amount of files required for Simplecov to function
3. Run the scripts/merge-simplecov script from any job as fast as possible (see iteration 3 of #416630 (comment 1474093453))

What does this MR do and why?

In this MR, we make the scripts/merge-simplecov script independent of the gitlab-org/gitlab codebase as much as we can.

We also introduce the BUNDLE_ONLY env variable to replace BUNDLE_WITHOUT, as it will then only load the gems we need for the coverage jobs to work.

Results

• The pipeline is green, which means that:
  • Coverage was generated for each RSpec jobs (e.g. download artifacts for https://gitlab.com/gitlab-org/gitlab/-/jobs/4753874798)
    • If the folders inside the coverage folder of the artifacts look empty, look for hidden files!
  • Coverage reports were merged in rspec:coverage: https://gitlab.com/gitlab-org/gitlab/-/jobs/4753874811
    • This job displays a coverage score of 98.39%

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.