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 repoints every update-package-index workflow at the image that
matches its own platform (was previously pinned to
build-env-ubuntu:noble-4.4 / noble-4.5 regardless of platform).
All 12 workflows were triggered simultaneously by the update-package-index cron. Now they run one after another via depends_on. Individual cron triggers still allow running one at a time.
