fix(cdn): purge the staging zone too (#181)

## Motivation

`cran-rpkgs-test` was added as a second pull zone on the same B2 origin, but it was never added to `BUNNY_PULLZONES`. It therefore keeps serving pre-reindex indexes behind the same ~370 day `cache_expiration_time` as production, and nothing ever refreshes it.

That is not cosmetic. The zone exists to be measured, and a verification run against it measures whatever the edge still holds:

```
production: regressions=0    AGHmatrix Path=NA   Built=R 4.5.3; x86_64-pc
staging   : regressions=161  AGHmatrix Path=4.5  Built=(none)
```

Same objects, same origin, 161 phantom regressions. I chased that number through two wrong diagnoses before noticing production and staging disagreed.

## Change

Add `cran-rpkgs-test.b-cdn.net` to the purge list in both reindex pipelines.

## Note

A `Cache-Control: no-cache` request header is not a substitute. It was added to the verification script and did **not** clear this: bunny does not honour it for these objects. Purging is the mechanism that works.

Reviewed-on: #181

.crow/reindex.yaml

@@ -172,7 +172,12 @@ steps:
         from_secret: BUNNYNET_API_KEY
       # cran.rpkgs.com and cran.allianceswisspass.devxy.io are on separate
       # Bunny pull zones, so both must be purged after the shared origin changes.
-      BUNNY_PULLZONES: '3857050 cran.allianceswisspass.devxy.io'
+      # The staging zone is listed too. It shares the B2 origin, so an index
+      # it still holds is a stale copy of the same object, and its
+      # cache_expiration_time is the same ~370 days: without a purge here it
+      # serves pre-reindex indexes indefinitely and any verification run
+      # against it measures the past.
+      BUNNY_PULLZONES: '3857050 cran.allianceswisspass.devxy.io cran-rpkgs-test.b-cdn.net'
     commands:
       - apk add --no-cache -q bash curl jq
       # Crow carries the checkout from the re-index step into this step.

.crow/weekly-rebuild-reindex.yaml

@@ -175,7 +175,12 @@ steps:
         from_secret: BUNNYNET_API_KEY
       # cran.rpkgs.com and cran.allianceswisspass.devxy.io are on separate
       # Bunny pull zones, so both must be purged after the shared origin changes.
-      BUNNY_PULLZONES: '3857050 cran.allianceswisspass.devxy.io'
+      # The staging zone is listed too. It shares the B2 origin, so an index
+      # it still holds is a stale copy of the same object, and its
+      # cache_expiration_time is the same ~370 days: without a purge here it
+      # serves pre-reindex indexes indefinitely and any verification run
+      # against it measures the past.
+      BUNNY_PULLZONES: '3857050 cran.allianceswisspass.devxy.io cran-rpkgs-test.b-cdn.net'
     commands:
       - apk add --no-cache -q bash curl jq
       # Crow carries the checkout from the re-index step into this step.