Merge #7070

7070: Improve cross repo integration r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that we had on `rubygems`, we're not testing bundler against the rubygems checkout of each PR, but against master. That is confusing because developers assume that the PRs tests will run against the PRs code, but this is currently not true.

### What was your diagnosis of the problem?

My diagnosis was that on rubygems we are [specifying `RGV=master`](https://github.com/rubygems/rubygems/blob/fa6e547330e97b417ed11f262d68d03c57abeeda/.travis.yml#L12) in our TravisCI matrix. And what `bundler` does with that is to clone rubygems master under `tmp/rubygems` and use that. Thus your rubygems PR is not really tested.

### What is your fix for the problem, implemented in this PR?

My fix is to rework the setup to accept an `RGV` environment variable when it contains. In that case, no git operations will be performed, and that path will be used directly and assumed to contain a rubygems checkout.

I also refactored a few things about the setup while at it.

### Why did you choose this fix out of the possible options?

There were existing tasks to test bundler against a rubygems checkout, but they were unused and they expected a `RG` environment variable instead. I decided to reuse these tasks, but make them pick up the `RGV` as long as it contains a valid path. I chose this fix because it works and it doesn't make the existing setup too different.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

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