Fix 'could not save history to file "//.psql_history"' errors

roles/matrix-postgres/templates/env-postgres-psql.j2

@@ -1,4 +1,8 @@
 #jinja2: lstrip_blocks: "True"
 PGUSER={{ matrix_postgres_connection_username }}
 PGPASSWORD={{ matrix_postgres_connection_password }}
-PGDATABASE={{ matrix_postgres_db_name }}
+PGDATABASE={{ matrix_postgres_db_name }}
+# Prevent errors like this:
+# > could not save history to file "//.psql_history": Permission denied
+# .. due to Postgres not being able to write to the filesystem.
+PSQL_HISTORY=/dev/null