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

@@ -158,11 +158,23 @@ steps:
   - name: Check which R versions 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
       - |
-          LISTING=$(curl -s "https://s3.eu-central-003.backblazeb2.com/devxy-r-builds?prefix=${PLATFORM_ID}/" | \
+          # 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-versions-existing.txt stays empty,
+          # and every version is rebuilt even though it already exists.
+          LISTING=$(curl -s --aws-sigv4 "aws:amz:eu-central-003:s3" \
+            --user "${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')
           : > r-versions-existing.txt
           for VERSION in $(cat r-versions-to-build.txt); do