Clean up CI code from before we used Github Actions.

[kousu]

• Use Ubuntu's git-annex instead of conda-forge's
  • Now that we're on Github Actions, we can rely on ubuntu-latest meaning "Ubuntu 20.04"
• Use more direct and more accurate labels
  • the old echos were a TravisCI hack
• Change the workflow's overall name to reflect reality.
• Use annex.thin instead of rm -rf to save space
  • but I left the rm -rf in in case we need to bring it back some day; I checked and Actions boots with about 20G of free space, and that rm -rf increases that to 50G.
  • Use bids-validator --verbose to get full details
    • It's logged by github, so there's no harm in saving as much information as possible

[kousu]

Just gotta wait to test it a bit before I submit this for final review.

[kousu]

Okay this is working now!