Move from CircleCI/Packet.net to GH Actions

[arm4b]

At the moment we build ST2 vagrant box + OVA by requesting a bare-metal instance at Packet.net, making a Packer build there and deploying produced artifact to the Vagrant Cloud.

Turns out GH Actions can now support Vagrant + Virtualbox (via nested virtualization? ).

Explore/research the https://stackoverflow.com/a/60380518/4533625 and https://github.com/jonashackt/vagrant-github-actions and moving the pipeline to GH Actions which will save us some $ and complexity by not using external Packet.net bare-metal instance. Side effects could be: slow/unstable build process, needs evaluation.

[winem]

I ran into an issue when releasing 3.6.0 because the Ubuntu 18.04.5 image was no longer available.

I'd vote to schedule the GitHub Actions execution in addition to the Push/PR-Triggers. Once a week is probably a sane value that gives us more visibility if there are any changes on dependencies without generating too much noise.

[arm4b]

Based on the latest v3.7.0 production build https://circleci.com/gh/StackStorm/packer-st2/4282, our Packet.net CI doesn't work anymore. This means we've failed to produce the latest Vagrant box for v3.7.0.

For reference, https://github.com/StackStorm/st2vagrantdev/blob/master/.github/workflows/ci.yaml has examples of successfully using Vagrant + Virtualbox in GH Actions.
Based on https://github.com/StackStorm/st2vagrantdev/blob/588fb96ed22c7ac9539ef30f25a439377c877475/.github/workflows/ci.yaml#L66-L67 nested virtualization is supported in the Mac OS X env.

We need to migrate the CI/CD here to GH Actions.
Help wanted.

[arm4b]

Closed via #64 by @mamercad