fix(ci): gate the Built-stamp repair on its own variable

The pipeline gated on `target_arch`, which is the same variable
build-all-versions and build-all-versions-install-deps gate on. A manual run
passing `target_arch=arm64` to reach the repair therefore also matched both of
those and started a full arm64 rebuild alongside it.

Every other pipeline here gates on a variable named after itself
(`process_cran_updates`, `weekly_audit_missing`, `weekly_rebuild_missing`), so
follow that and gate on `repair_built_stamp`. Record the collision and the exact
invocation in the header so the next pipeline added here does not repeat it.

.crow/repair-built-stamp.yaml

@@ -9,9 +9,18 @@
 # Run with `dry_run: true` first: it reports how many entries are broken per
 # slot and changes nothing. Pick the R version the slot should advertise, which
 # is the R_VERSION its entry in .crow/process-updates.yaml uses.
+#
+# The gate variable is `repair_built_stamp`, not `target_arch`: `target_arch` is
+# what build-all-versions and build-all-versions-install-deps gate on, so a
+# manual run passing it would start a full rebuild alongside this repair. Every
+# pipeline here gates on a variable named after itself for exactly that reason.
+#
+#   crow pipeline create --branch main \
+#     --var repair_built_stamp=arm64 --var OS=alpine --var OS_VERSION=3.22 \
+#     --var R_VERSION=4.5.3 --var dry_run=true devxy/build-cran-binaries
 variables:
-  target_arch:
-    description: 'Architecture of the slot to repair.'
+  repair_built_stamp:
+    description: 'Architecture of the slot to repair. Also gates this pipeline.'
     options:
       - amd64
       - arm64
@@ -51,7 +60,7 @@ variables:
 
 when:
   - event: manual
-    evaluate: 'target_arch == "${ARCH}"'
+    evaluate: 'repair_built_stamp == "${ARCH}"'
 
 skip_clone: true