pmgpolicy, pmg-smtp-filter: set sensible PATH

pmgpolicy, pmg-smtp-filter: set sensible PATH

these 2 services are the only ones that don't have this set (the
additions are copied over from pmgproxy)

with bookworm pmgpolicy throws an error (when invoking 'journalctl'
via run_command (in PMG::Utils::scan_journal_for_rbl_rejects)

did not come to a conclusion why it just resurfaced now, and ran fine
in a bullseye environment

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>