trivial: snap: save artifacts from build
trivial: snap: save artifacts from build

Should help for debugging problems without a local build as well
as in case upload to snapcraft.io fails to avoid having to re-run
full build.

fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd