fix(ci): gate the three ungated pipelines on their own variable

A manual pipeline creation instantiates every pipeline in .crow/, so one whose
only manual condition is a bare `event: manual` fires on any manual trigger in
this repo. auto-apply-patches, weekly-patch-proposals and trial-build-registry
were in that state and all ran unintentionally alongside a manual
process-updates run.

- gate each on a variable named after the pipeline, defaulting to 'false', as
  repair-built-stamp.yaml documents and the other pipelines already do
- refresh the stale run-manually comments, which still referenced a `task`
  variable that no pipeline evaluates

.crow/trial-build-registry.yaml

@@ -7,15 +7,27 @@
 #
 # The repo uses no `pull_request` triggers, so this runs manually against the
 # branch (or on a cron); point it at the auto-patch branch via `patch_branch`:
-#   woodpecker-cli pipeline create --var task=trial-build-registry \
-#     --var patch_branch=auto/registry-patch-proposals --branch=main 7
+#   crow pipeline create --branch main --var trial_build_registry=true \
+#     --var patch_branch=auto/registry-patch-proposals devxy/build-cran-binaries
+#
+# The gate variable is `trial_build_registry`, named after the pipeline: a
+# manual run instantiates every pipeline in `.crow/`, so one without its own
+# gate runs on *any* manual trigger in this repo. This one starts a build per
+# matrix row on both arches, which is far too expensive to fire by accident.
 variables:
+  trial_build_registry:
+    description: 'Trial-build the branch new registry entries. Also gates this pipeline.'
+    options:
+      - 'true'
+      - 'false'
+    default: 'false'
   patch_branch:
     description: 'Branch whose new registry entries to trial-build.'
     default: auto/registry-patch-proposals
 
 when:
   - event: manual
+    evaluate: 'trial_build_registry == "true"'
   - event: cron
     cron: trial-build-registry