gitlab-ci: Allow doc-tarball job to fail

Currently the Hadrian build appears not to package documentation correctly,
causing doc-tarball to fail due to the Windows build.

.gitlab-ci.yml

@@ -981,6 +981,8 @@ release-x86_64-windows-integer-simple:
 doc-tarball:
   stage: packaging
   needs: [validate-x86_64-linux-deb9-debug, validate-x86_64-windows-hadrian, validate-x86_64-linux-deb9-unreg-hadrian]
+  # N.B. Documentation isn't correctly packaged in Hadrian bindists
+  allow_newer: true
   tags:
     - x86_64-linux
   image: "registry.gitlab.haskell.org/ghc/ci-images/x86_64-linux-deb9:$DOCKER_REV"