Drop the R-version suffix from each image tag, add an explicit
R_VERSION env var per file, and invoke R via
/opt/R/${R_VERSION}/bin/R at every call site.
Also aligns ubuntu-2404 process-updates from R 4.4 to R 4.4.3,
matching the audit and rebuild counterparts.
Both pipelines had platform = 'alpine-321' instead of their actual target, causing builds to upload to the wrong S3 path and the PACKAGES index to not reflect newly built packages.
Each process-updates workflow now calls upload_package_index at the end to update the PACKAGES files for its platform/arch immediately after builds complete. The full update-package-index cron should be rescheduled to weekly.
