Add Gemfile.checksum for rspec:coverage job

From Draft to Ready

• Remove the test commit !128744 (ad5d4672)

Context

We received a report of a pipeline failing because of the rspec:coverage job (see MR - see failing job)

What does this MR do and why?

Download the Gemfile.checksum for the rspec:coverage job.

When updating gems in an MR, we will be writing locally to the Gemfile.checksum.

Since we do not download this file in the no git clone jobs, bundler will complain that the file does not exist, and errors out.

Does it work?

To reproduce the failure in !127896 (diffs), I added a commit (!128744 (ad5d4672)) that updates a gem like in the above MR.

The rspec:coverage job is green with that commit ✅

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.