produce source tarballs

I see that our pipelines appear to produce artifacts that contain a pre-built x86_64 binary tarball that can just be unpacked into /usr/local.

It seems to me like FreePG might also want to emit a "tarball" of some sort for each tag.

If we want to emit a tarball the way that GnuPG upstream does, i think it would be along these lines:

./autogen.sh --force
./configure --sysconfdir=/etc --enable-maintainer-mode
make dist

This bundles in a bunch of autotools cruft, since GnuPG has a sort of legacy distribution process. maybe we don't want to do that (the autotools cruft that shipped in xz masked the malware in the xz backdoor, for example).

If we'd rather do something different, we might want to use git archive to generate a tarball, but that would require filtering out a lot of the cruft that has been committed to the git repository over the years as well. It looks like we could use the export-ignore git attribute for that, but if we go that route someone™ will need to sort out which files should get the attribute.