chore: CI: Create an all-builds-ok job (#2983)

there is a little dance with `if: success()` because otherwise a failed
`build` job would make this new job skipped, not failed, and I fear
skipped means ok when it is a required job.
So let’s make sure this job actually fails.

.github/workflows/ci.yml

@@ -364,6 +364,21 @@ jobs:
             ./build/stage2/bin/lean
             ./build/stage2/lib/lean/libleanshared.so
 
+  # This job collects results from all the matrix jobs
+  # This can be made the “required” job, instead of listing each
+  # matrix job separately
+  all-done:
+    name: Build matrix complete
+    runs-on: ubuntu-latest
+    needs: build
+    if: ${{ always() }}
+    steps:
+    - if: contains(needs.*.result, 'failure') ||  contains(needs.*.result, 'cancelled')
+      uses: actions/github-script@v3
+      with:
+        script: |
+            core.setFailed('Some jobs failed')
+
   # This job creates releases from tags
   # (whether they are "unofficial" releases for experiments, or official releases when the tag is "v" followed by a semver string.)
   # We do not attempt to automatically construct a changelog here: