The existing-version check listed the bucket with an anonymous curl, but
Backblaze B2 requires authentication for the list-bucket API, so it returned
AccessDenied and r-versions-existing.txt was always empty -> every version
was rebuilt despite already existing in s3.
Signing the request is not workable here: the B2 credentials are scoped to
the plugin-s3 image in crow and are not injected into this plain alpine step
(curl saw --user ":").
Individual artifacts are public-read, so probe each expected object URL with
an anonymous HEAD instead of listing the bucket. The filename mirrors what
nfpm produces (version/release pinned to 1): r-<ver>_1_<arch>.{apk,deb} for
alpine/ubuntu and R-<ver>-1-1.<arch>.rpm for el*. Verified against the live
bucket for all three packager families.
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.
Reviewed-on: #7
