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 8f5520c3)