Re-add stages

GitLab docs explicitly say `needs` does not require stages but GitLab actual behavior is:

```
Found errors in your .gitlab-ci.yml:

    update-staging job needs system-tests job, but system-tests is not in any previous stage
    update-staging job needs morph-build-staging job, but morph-build-staging is not in any previous stage

```

.gitlab-ci.yml

@@ -18,12 +18,17 @@
 
     - when: "never"
 
+stages:
+  - "build"
+  - "deploy"
+
 default:
   # Guide the choice of an appropriate runner for all these jobs.
   # https://docs.gitlab.com/ee/ci/runners/#runner-runs-only-tagged-jobs
   tags:
     - "nixos"
     - "shell"
+  stage: "build"
 
 variables:
   # https://docs.gitlab.com/ee/ci/runners/configure_runners.html#job-stages-attempts