fix(ci): authenticate B2 list-bucket request in existing-version check

Backblaze B2 requires authentication for the list-bucket API; anonymous
access only works for individual public-read objects. The unauthenticated
curl returned AccessDenied, so r-version-s3.txt was always empty and every
R version was rebuilt despite already existing in s3.

Sign the listing request with --aws-sigv4 using the same B2 credentials as
the upload step. Also fix the broken version extraction: the old
substr($NF, 3, 5) ran against the prefixed key and yielded garbage, so the
Build step's grep could never match. Print the actual version on a match
and use a literal index() instead of a regex comparison.

.crow/build.yaml

@@ -139,15 +139,27 @@ steps:
   - name: Check if files for R version already exist in s3
     image: reg.devxy.io/docker.io/library/alpine:3.23
     privileged: true
+    environment:
+      AWS_ACCESS_KEY_ID:
+        from_secret: B2_S3_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: B2_S3_SECRET_KEY
     commands:
       - ip link set dev eth0 mtu 1280 2>/dev/null || true
       - for i in 1 2 3 4 5; do apk add -q --no-cache curl && break; sleep 5; done
       - |
+          # Backblaze B2 requires authentication for the list-bucket API (anonymous
+          # GET works only for individual public-read objects), so the request must be
+          # SigV4-signed with the same credentials used for the upload step. Without
+          # this the listing returns AccessDenied, r-version-s3.txt stays empty, and
+          # every version is rebuilt even though it already exists.
           VERSION=$(cat r-version-to-build.txt)
-          curl -s "https://s3.eu-central-003.backblazeb2.com/devxy-r-builds?prefix=${PLATFORM_ID}/" | \
+          curl -s --aws-sigv4 "aws:amz:eu-central-003:s3" \
+            --user "${AWS_ACCESS_KEY_ID}:${AWS_SECRET_ACCESS_KEY}" \
+            "https://s3.eu-central-003.backblazeb2.com/devxy-r-builds?prefix=${PLATFORM_ID}/" | \
             grep -oE '<Key>[^<]+</Key>' | sed 's/<[^>]*>//g' | \
             awk -v arch="${ARCH_ID}" -v ver="${VERSION}" \
-              '$0 ~ arch && $0 ~ ver {print substr($NF, 3, 5)}' > r-version-s3.txt
+              'index($0, arch) && index($0, ver) { found = 1 } END { if (found) print ver }' > r-version-s3.txt
       - cat r-version-s3.txt
     backend_options:
       kubernetes: