Not strict enough with pristine-tar

The .l5-check-tarball job does not match what the PureOS builder will later do with pristine-tar.

The particular issue that this job does not fail is the archive file is plain TAR with an .tar.xz ending: https://source.puri.sm/dorota.czaplejewicz/deb-libcamera/-/jobs/392117

PureOS will reject such jobs: https://lists.community.puri.sm/pipermail/librem5-builds/2022-July/003817.html

unxz: (stdin): File format not recognized
tar: This does not look like a tar archive