Migrate runners

[njooma]

From Slack, we should move our workflows away from self-hosted runners to either github or buildjet hosted:

github runners start up faster (we host the rdk-devenv image on github, so it's faster for it to pull the image) so are best for small jobs, and better than doing a 2-core runner on buildjet. Anything that really needs 4 cores or more than 4GB of ram should use buildjet probably, and scale appropriately. Typically 4 or 8 core is the right answer. I've not seen a job that can really make full use of 16 cores for a full run (though if you've got one, then it's worth considering.)