fix(rebuild): harden split workflow setup (#164)

## Motivation

Weekly rebuild shards can all hit a transient CRAN DNS/index outage at once, and the dependent CDN purge always fails because it tries to clone over the checkout preserved from the re-index step.

## Changes

- Retry `uvr add` resolution up to four times with bounded backoff.
- Reuse the existing Crow workspace checkout in the CDN purge step.
- Remove the purge step's unused Git package and repository token.

## Validation

- `crow lint .crow/`
- `shellcheck local/uvr-install.sh scripts/purge_cdn_zone.sh`
- `git diff --check`

Reviewed-on: #164

.crow/weekly-rebuild-reindex.yaml

@@ -173,14 +173,12 @@ steps:
       OTEL_R_METRICS_EXPORTER: none
       BUNNYNET_API_KEY:
         from_secret: BUNNYNET_API_KEY
-      REPO_RO_TOKEN:
-        from_secret: REPO_RO_TOKEN
       # All hostnames on the zone share this id, so one purge covers
       # cran.devxy.io, cran.allianceswisspass.devxy.io and cran.rpkgs.com.
       BUNNY_PULLZONE: '3857050'
     commands:
-      - apk add --no-cache -q bash curl git
-      - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
+      - apk add --no-cache -q bash curl
+      # Crow carries the checkout from the re-index step into this step.
       - bash scripts/purge_cdn_zone.sh "$BUNNYNET_API_KEY" "$BUNNY_PULLZONE"
     # Runs on every row rather than on one designated slot: a cron fires only
     # its own slot's row, so gating on a named slot would leave every other