meson: initialize time-epoch to reproducible builds compatible value

meson: initialize time-epoch to reproducible builds compatible value

Debian Policy encourages to preserve timestamps whenever possible in the
tarballs, thus stable release updates of systemd usually do not bump NEWS file
timestamp. And thus time-epoch remains the same for the lifetime of a release.

It would be better, if each new stable release rebuild of systemd would bump
the time epoch a bit. But at the same time remain
reproducible. SOURCE_DATE_EPOCH is an environmnet variable defined for this
purpose. Thus if available, prefer that, instead of the NEWS file modification
time.

For example, on Debian/Ubuntu under the reproducible builds the
SOURCE_DATE_EPOCH is set to the timestamp from the packaging metadata, thus it
is incremented on every new stable release update, whilst preserving
reproducible builds capability.

Reference: https://reproducible-builds.org/docs/timestamps/
(cherry picked from commit 6dbf352cfbbaf9c9b277af54da50da38296ae5c6)