Convert zip build and upload into GitHub Action

[Rarst]

Creating and attaching zip build is automated in Robo command, however since it's based on Composer create-project (so any local changes aren't dragged in, etc) there is a lag between pushing to GitHub and version appearing on Packagist to wait for.

I think GitHub Actions have release-related triggers now, so that should be (finally) refactored to run there and build from clean checkout, w/o involving Packagist.