r/wal-g-pg: Run wal-g backup as postgres

`wal-g` needs to connect to the PostgreSQL database system, so it should
run as the _postgres_ user, who has permission to connect, rather than
_root_, who does not.

roles/wal-g-pg/files/wal-g-backup.service

@@ -7,3 +7,4 @@ After=network-online.target
 Type=oneshot
 Environment=PGDATA=/var/lib/pgsql/data
 ExecStart=/usr/bin/wal-g-pg backup-push --config /etc/postgresql/wal-g.yml $PGDATA
+User=postgres