ci: Add `dist-then-build` target to catch missing `EXTRA_DIST`

ci: Add `dist-then-build` target to catch missing `EXTRA_DIST`

I personally am very opposed to the entire idea of `make distcheck`;
I think source code should canonically be *git* and not tarballs.
See e.g. https://github.com/cgwalters/git-evtag for some
rationale.

But anyways we are uploading classic tarballs since today that
what Debian/Fedora/etc consume sadly, so we need to test it.

We explicitly skip `make distcheck` since we don't want to rerun
the test suite.

Closes: #1766
Approved by: jlebon