fix(ci): gate the Built-stamp repair on its own variable (#151)

## Problem

`repair-built-stamp.yaml` gated on `target_arch`:

```yaml
when:
  - event: manual
    evaluate: 'target_arch == "${ARCH}"'
```

That is the same variable `build-all-versions.yaml` and `build-all-versions-install-deps.yaml` gate on. A manual run passing `target_arch=arm64` to reach the repair therefore matched all three, so triggering a dry-run repair also queued a full arm64 rebuild.

I hit this triggering the alpine 3.22 dry run (pipeline 10706), which I killed. `crow pipeline ps` renders empty states on this version, so I could not confirm from the CLI whether the rebuild workflows started before the kill or only sat queued behind the running cron jobs; no output was attributable to them.

## Change

Gate on `repair_built_stamp` instead. Every other pipeline here already gates on a variable named after itself (`process_cran_updates`, `weekly_audit_missing`, `weekly_rebuild_missing`), and `target_arch` was the odd one out being shared by two.

The header now records the collision and the exact invocation, so the next pipeline added here does not repeat it.

## Verification

`crow lint` passes; `prek` clean.

Grep of every trigger condition in `.crow/` confirms `repair_built_stamp` is unique and that no other pipeline gates on `OS`, `OS_VERSION`, `R_VERSION` or `dry_run` alone:

```
archive-missed-packages.yaml:  task == "archive-missed-packages"
build-all-versions.yaml:       target_arch == "${ARCH}"
build-all-versions-install-deps.yaml: target_arch == "${ARCH}"
weekly-audit-missing.yaml:     weekly_audit_missing == ...
repair-built-stamp.yaml:       repair_built_stamp == "${ARCH}"
process-updates.yaml:          process_cran_updates == ...
weekly-rebuild-missing.yaml:   weekly_rebuild_missing == ...
```

## Note for whoever runs it

`crow pipeline create` against this instance returned HTTP 504 while still creating the pipeline. Retrying on that error duplicates the run: I created four before noticing. Verify with `pipeline ls` rather than trusting the exit status.

Reviewed-on: #151

.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