* guix/scripts/lint.scm (check-source-unstable-tarball): New procedure. (%checkers): Add it. * tests/lint.scm ("source-unstable-tarball", "source-unstable-tarball: source #f", "source-unstable-tarball: valid", "source-unstable-tarball: package named archive", "source-unstable-tarball: not-github", "source-unstable-tarball: git-fetch"): New tests. * doc/guix.texi (Invoking guix lint): Document it.