GitHub Actions CI now enabled for INN pull requests

I've enabled some simple GitHub Actions testing for all pull requests and
pushes for the main branch.  This does the same checks that we are doing
when generating snapshots: builds with various options with warnings
enabled plus running the (somewhat meager, alas) test suite.  That should
at least give us a basic sanity check for pull requests.

(Sorry about the noise in inn-committersts earlier.  I hadn't realized
that branches in the repository would also generate notifications, and was
iterating on fixing issues that were only showing up on the GitHub Ubuntu
runners.)

-- 
Russ Allbery ([email protected])             <https://www.eyrie.org/~eagle/>

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