tests: one run per commit, and nothing from apt

.github/workflows/test.yml

@@ -1,4 +1,4 @@
-# Verify factory on every push and PR.
+# Verify factory on every push to main, and on every PR.
 #
 # What goes wrong with these scripts is not a crash. `factory shift` runs
 # unattended and its product is a log read hours later, so its worst output is a
@@ -43,21 +43,44 @@ name: Tests
 
 on:
   push:
-    branches: ['**']
+    branches: [main]
   pull_request:
 
+# A second push to a PR branch makes the first run's answer worthless, and the
+# abandoned run still holds a runner — a runner the next PR then waits
+# behind.
+#
+# Only a PR run is ever in a shared group. Everything else keys on the commit,
+# so it gets a group of its own: putting main pushes in one group would
+# SERIALIZE them even with cancelling off, and GitHub cancels a *pending* run
+# when a newer one joins its group — so three merges inside one build would
+# leave the middle commit with no run at all. That is the tick this is
+# supposed to protect.
+concurrency:
+  group: tests-${{ github.event_name == 'pull_request' && github.ref || github.sha }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   test:
     name: bats + shellcheck + skill guards
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
 
-      # jq is named rather than assumed present: the tier filter IS a jq
-      # program and the config layer is another, so a runner image that dropped
-      # it would turn every case into an error rather than a failure.
-      - name: Install bats + shellcheck + jq
-        run: sudo apt-get update && sudo apt-get install -y bats shellcheck jq
+      # bats is the one tool of the three the ubuntu-latest image lacks, and
+      # npm is preinstalled, so it comes from npm rather than apt. shellcheck
+      # and jq are already on the image; `apt-get update` in front of them
+      # bought nothing and, on the day the Azure mirrors go quiet, hangs rather
+      # than fails (haus/check.yml has the long version of that afternoon).
+      #
+      # jq is still named rather than assumed: the tier filter IS a jq program
+      # and the config layer is another, so an image that dropped it would turn
+      # every case into an error rather than a failure. The version print below
+      # is what makes that loud instead of confusing.
+      - name: Install bats
+        run: sudo npm install -g bats
+      - name: jq is on this image
+        run: jq --version
 
       # The versions are printed because they are not pinned, and apt's
       # shellcheck disagrees with a newer one about which checks are on by