Arch Linux ARM's makepkg.conf defaults PKGEXT to .pkg.tar.xz. build/sign.sh
only signs *.pkg.tar.zst, and push-build, sync-rebuilds and the notifier
parse the same suffix, so an aarch64 package built under the stacked
pipeline came out as .xz and would have been skipped at signing. Seen on a
plain x86_64 runner building aarch64 under QEMU (fork run 33642125077).
Same fix as the PKGEXT line in #240.
Marcelo Alcantara
b677f50358