gh/actions unit test workflows

[JamesKunstle]

Adds a workflow to run our unit tests via tox on an EC2 runner. Runs:

1. When a PR is opened, synchronized, and re-opened.
2. When there is a push to the main or 'release-' branches.

[nathan-weinberg]

@courtneypacheco @bjhargrave could y'all PTAL at this as well?

[courtneypacheco]

Had a few questions besides Nathan's, but looks good otherwise!

[JamesKunstle]

@danmcp @nathan-weinberg @bjhargrave Could you please look at the failure that the job is getting and help figure out why the workflow can't access vars.AWS_REGION? I've played around with it for a while and I can't figure out why the vars context doesn't have anything in it. I'm adding a debugging step to show that the variable is indeed empty.

[danmcp]

@danmcp @nathan-weinberg @bjhargrave Could you please look at the failure that the job is getting and help figure out why the workflow can't access vars.AWS_REGION? I've played around with it for a while and I can't figure out why the vars context doesn't have anything in it. I'm adding a debugging step to show that the variable is indeed empty.

It's not going to be able to access the variable from a fork PR with a new workflow.

[JamesKunstle]

@danmcp Oh, duh. Of course it can't.