CI: Switch back to using 'dependencies'

Using 'needs' broke CI:
https://gitlab.haskell.org/ghc/head.hackage/-/pipelines/57782

'dependencies' doesn't care if the dependency exists or not, but 'needs'
does.

.gitlab-ci.yml

@@ -236,7 +236,7 @@ update-repo:
     - nix run -f ci/default.nix -c build-repo.sh extract-keys
     - nix run -f ci/default.nix -c build-repo.sh build-repo
 
-  needs:
+  dependencies:
     - build-master
 
   after_script:
@@ -254,7 +254,7 @@ pages:
   image: "nixos/nix:$DOCKER_TAG"
   script:
     - mv repo public
-  needs:
+  dependencies:
     - update-repo
   rules:
     - if: '$CI_COMMIT_BRANCH == "master"'