meson: Ensure that distribution packages own systemenvgeneratordir

meson: Ensure that distribution packages own systemenvgeneratordir

Currently, Fedora's systemd RPM doesn't own systemenvgeneratordir
(ie., /usr/lib/systemd/system-environment-generators) [1] because it's
not created when systemd is installed.  In contrast, userenvgeneratordir
(ie., /usr/lib/systemd/user-environment-generators) is created, unless
the environment-d Meson option is explicitly disabled.

While this can be worked around elsewhere, it's better if the upstream
build system created the directories consistently.  It will avoid
repetition, and prevent silly bugs or deviations from creeping in.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=2284085