feat: add cron task that runs the minimal training pipeline nightly

.cron.yml

@@ -0,0 +1,12 @@
+jobs:
+  # Run the minimal pipeline once a day to support integration testing before
+  # worker image changes are made (see https://bugzilla.mozilla.org/show_bug.cgi?id=1937882).
+  # These runs will pick up cached tasks, so most of the time this will simply
+  # end up running `all-pipeline`.
+  - name: run-pipeline
+    job:
+      type: trigger-action
+      action-name: train
+      include-cron-input: true
+    when:
+      - {hour: 0, minute: 0}