postgresql: wal-g: Set PGHOST

By default, WAL-G tries to connect to the PostgreSQL server via TCP
socket on the loopback interface.  Our HBA configuration requires
certificate authentication for TCP sockets, so we need to configure
WAL-G to use the UNIX socket.

group_vars/postgresql.yml

@@ -49,3 +49,4 @@ wal_g_pg_config:
   WALG_S3_PREFIX: s3://pgbackup/pyrocufflink/main/15
   AWS_S3_FORCE_PATH_STYLE: true
   AWS_ENDPOINT: https://burp.pyrocufflink.blue:9000
+  PGHOST: /run/postgresql