ci: Add gather step

We can require a status to be successful in PRs, but they are
referenced by name, and so we'd have to add each matrix job as
required. That's rather cumbersome, so have this artificial gather
step at the end which signals success on the entire run.

.github/workflows/ci.yaml

@@ -243,3 +243,17 @@ jobs:
         run: |
           tar -xf cln-${CFG}.tar
           poetry run pytest tests/ -n ${PYTEST_PAR} ${PYTEST_OPTS}
+
+  gather:
+    # A dummy task that depends on the full matrix of tests, and
+    # signals successful completion. Used for the PR status to pass
+    # before merging.
+    name: CI completion
+    runs-on: ubuntu-20.04
+    needs:
+      - integration
+      - check-units
+    steps:
+      - name: Complete
+        run: |
+          echo CI completed successfully