[build2] build2 Action for GitHub Workflows

[Boris Kolpackov]

Fabian Meyer <fabian.meyer at posteo.de> writes:

> I also pushed a v2 tag, however for publishing the tag to the GitHub
> marketplace I would need a build2 organization admin to accept the "GitHub
> Marketplace Developer Agreement".
> I am lacking the admin rights, so I cannot do it.

I've made you the admin of this repository, can you check if you are
now able to do this?

BTW, is the name of the repository (setup-build2) somehow important
to the GitHub Actions integration? If not, do you think we can rename
it to something more specific, like github-actions-setup-build2 or
some such? We have quite a few build2-related repositories in this
organization and just setup-build2 is a bit too generic.