Switch from buildkite to GitHub actions.

[plietar]

We now have the ability to connect to Packit directly from GitHub actions, which removes the need to use our internal Buildkite infrastructure.

The new workflow is simpler by not having to deal with Docker, and should also be easier to debug and work with. It uses the new mrc-ide/orderly-action to handle the initialization and authentication to Packit.

[plietar]

This works similarly to the existing buildkite pipeline: it reads the output of db_extraction from Packit and runs the rest of the workflow. The packets it produces are pushed back to the Packit server.

Unfortunately the analysis based on the db_extraction packet that is hosted on Packit is broken and the workflow fails to complete. I tried to re-run the db_extraction task locally to regenerate that packet but ran into encoding issues when reading from the Access database.