- Only publish the bz2 tarball because that's what we currently provide
  on download.gimp.org (let's see in the future for the xz tarball).
- Generate also a sha512 checksum. It's better to do it on the CI rather
  than on the download server, otherwise it wouldn't protect against
  transfer errors (from gitlab to download server).
- Rename the checksum files to ${filename}.SHA256SUMS (512 respectively)
  for easy download without name clash with the global files listing all
  the previous releases.
- Disable all meson jobs for tagged releases. They are currently not
  reliable and may fail randomly (see issue #6257), even though without
  code problems (this is also one of the reasons why autotools is still
  our official build system and meson is still deemed experimental).
  It's ok for regular builds, but not for tagged builds, which we need
  as reliable as possible. The really important job is the distcheck one
  for tags.