Reproducible Builds: Do not embed user id and user name

rkward is currently embedding both the username and the userid in usr/share/rkward/rpackages/rkward.tgz and usr/share/rkward/rpackages/rkwardtests.tgz (as discovered by reproducible.archlinux.org. There doesn't seem to be a way to disable this with cmake -E tar, so this patch uses regular tar with the right flags.