We already support reproducible builds since commit v8.24-99-gc1b3d6587,
and this adjusts that change to also support reproducible
tarball contents with subsequent runs of `make dist`.
* Makefile.am: Don't create a varying .timestamp file, instead ...
* man/local.mk: Rely on the timestamp of the .tarball-version file.
Fixes https://bugs.gnu.org/72232