[RFC] CI: Ensure package updates can be checked (and if any updates are available)

[JamiKettunen]

Currently new packages can be submitted with broken ./xbps-src update-check's and I'd like to change that. I think it's important to have working update checking in packages, and as a bonus give some notice/warning when updates are available for packages on a PR when CI runs.

Chatlogs from #xbps IRC:

2022-07-23 00:45:43	deathmist	would it make sense for xlint to check package updates? I think it would be nice to verify new updates can be checked for a new package submitted to void-packages (and I guess fail CI if there's updates before even building?)
2022-07-23 00:50:07	deathmist	something like: [ -n "$(./xbps-src update-check $pkg 2>&1)" ] && (error etc. updates found or couldn't check them)
2022-07-23 00:50:55	abby	would be better as a different ci lint, I think (like the commit lint)
2022-07-23 00:51:40	abby	and not being the newest version isn't automatically bad, sometimes updates further can't/shouldn't be done
2022-07-23 00:51:42	deathmist	sure, but it would be useful yes? not sure about the failing on available updates part, perhaps it could emit some warning?
2022-07-23 00:56:07	abby	it might be good to check if the update-check works, but only error if the check is broken (no VERSION etc), not if further updates are available
2022-07-23 01:00:34	deathmist	abby: hm, not even a notice/warning (I don't know if that is possible on GH actions / travis without failing CI) when there's updates?
2022-07-23 01:01:11	deathmist	I guess a message could be printed if there's updates, but it'll get lost with the other CI logs if all is green
2022-07-23 01:01:51	deathmist	(at leas that's my concern)
2022-07-23 01:09:49	abby	maybe a notice or warning could work
2022-07-23 01:42:55	\\	abby, deathmist: github ci supports warnings/notices, so that'd work