databases/prometheus-postgres-exporter: Allow environment to be set

When not using multi-target mode, postgres_exporter relies on its
environment for configuration of the monitoring target. Add a new
postgres_exporter_env rc.conf option to allow this to be configured.

PR: 290694
Reviewed by: arrowd
Approved by: arrowd (ports)
Differential Revision: https://reviews.freebsd.org/D53513

(cherry picked from commit 8f5520c3bdaad14bead3d5644ea787787f906349)