Make template tar.bz2's reproducible.

This incorporates recommendations from https://reproducible-builds.org/docs/archives/ to better make tarballs reproducible. In some build environments like in Guix, the GID of "root" is not consistant.

With --numeric-owner, we only need to specify UID and GID, the user name is uneeded, so set --owner=0 --group=0

Currently, I observed a bug where the mtime of directories was set to the current time, breaking reproduciblity, so set --mtime as per recommendation. Note that SOURCE_DATE_EPOCH does need to be set for this to work. @ means the time will be intepreted as Unix Epoch.

Delete atime and ctime completely. No idea if this is better or worse.

Note: I'm not the original author, but the patch fixes the mtime issues I'm seeing on Arch. For example https://reproducible.archlinux.org/api/v0/builds/225293/diffoscope

Patch originally from https://bugs.kde.org/show_bug.cgi?id=443532 BUG: 443532

/cc @apol as per your request earlier, hope this MR helps move this forward Would be amazing if it can be backported to the stable branches.