ci-fairy: add upstream as custom-named repo

Sometimes ci-fairy's check-commits gets an invalid range of commits despite comparing the right branches. Often re-running the job fixes it, so it's not a bug in ci-fairy but probably caused by stale state.

Let's avoid any possible stale state in the upstream repo by creating a new repository per job, this way things like cifairy/main..HEAD should always do the right thing.

Example job failing and succeeding on re-run