Add boot testing to PRs

[mwasilew]

Boot testing was added in PR #96. This allows to boot test in nightly builds and pushes. There is a mechanism to allow for boot testing on PRs: https://securitylab.github.com/resources/github-actions-preventing-pwn-requests/. Based on this description it should be possible to add boot testing on PRs using workflow_run trigger.

[ndechesne]

Yes, this is correct, pull_request_target would allow us to do that. That's how we did it for meta-qcom in Linaro CI (https://github.com/Linaro/meta-qcom/blob/master/.github/workflows/premerge.yml).

[quic-kdybcio]

Make sure to only run these tests for approved runs, so that no one gets the funny idea of setting some voltage regulators too high on our CI farm ;)

[mwasilew]

I think physical damage to the hardware is least of my security concerns. Right now the tests are only scheduled for merges and nightly builds.