ci(postgres): install payments app on the test site

The Postgres CI site only listed erpnext in install_apps, so the payments app
(fetched and built by install.sh via 'bench get-app payments') was never
installed on the site — leaving 'tabPayment Gateway' absent. test_payment_request
(and other payment-gateway-dependent tests) then errored on Postgres with
'relation "tabPayment Gateway" does not exist', while MariaDB passed because its
site_config already lists ["payments", "erpnext"]. Match that ordering for parity.

.github/helper/site_config_postgres.json

@@ -13,6 +13,6 @@
  "root_login": "postgres",
  "root_password": "travis",
  "host_name": "http://test_site:8000",
- "install_apps": ["erpnext"],
+ "install_apps": ["payments", "erpnext"],
  "throttle_user_limit": 100
 }