Migrate Travis CI workflow to GitHub workflow

[ivandavidov]

Update the newly introduced manual GitHub workflow and implement the steps in the Travis CI workflow. Heartbeat functionality is not necessary, since GitHub workflow runners don't have limitation on the log length (at least for now) which was a great issue with Travis CI. Ensure that all tests are implemented (Docker test, QEMU test). No need to keep the generated ISO artifact.

[ivandavidov]

All integration tests pass.

https://github.com/ivandavidov/minimal/runs/3129943314?check_suite_focus=true

The only thing left is to redesign the workflow by splitting the steps in separate jobs (preparation step, build step, test step),