Upload kernel image to GitHub

[kloenk]

We could store the bzImage on GitHub, and just download from there to make testing easier: https://docs.github.com/en/free-pro-team@latest/actions/guides/storing-workflow-data-as-artifacts

[ojeda]

Trying this out, it seems uploading artifacts is unreliable: it failed for me twice in separate runs. It also makes the jobs fail, so for the moment, let's just avoid it -- we don't need it that much anyway.

See e.g. actions/upload-artifact#71