[PRA-264] Fix TIOBE workflow

[Batalex]

This PR fixes the broken TIOBE workflow that had gone unnoticed for a while.

The root cause was simple: we (and by "we", I mean "I") introduced a while ago a test using skopeo to check the compatibility between the various OCI resources involved.

Instead of installing the tool in the runner, I deselected this specific test since it does not contribute to the testing coverage and makes external network calls. I don't have the strongest of opinions here, so I'm happy to proceed the other way around and install skopeo on the runner.

You can check in the action tab that we now have a green run from the workflow (and even check the dashboard with a new quality measurement from today). Since I removed the commit in this PR running the workflow on pull request, the thing is a bit hidden.

[theoctober19th]

One way we can still keep the test and also not have to install skopeo in the runner machine is to install rockcraft snap in the runner, and use skopeo: str to provide rockcraft.skopeo value. I don't have strong opinion on keeping / removing the test from tox -e unit as long as it is possible to invoke this test somewhere separately in the CI (such that we don't leave gaps updating image with wrong version).

[Batalex]

One way we can still keep the test and also not have to install skopeo in the runner machine is to install rockcraft snap in the runner, and use skopeo: str to provide rockcraft.skopeo value. I don't have strong opinion on keeping / removing the test from tox -e unit as long as it is possible to invoke this test somewhere separately in the CI (such that we don't leave gaps updating image with wrong version).

Oh yes, we definitely could install the tool in the runner, whether it be through the rockcraft snap or the vanilla tool itself. We could also just skip the test if skopeo is not found in the PATH, which is something I would prefer not to do.