rbac: order columns for migration (sourcegraph#48001)

Adds an explicit order to the columns for the roles insert statement to improve robustness when running migrations up and down.

migrations/frontend/1671543381_add_default_roles/up.sql

@@ -1,6 +1,6 @@
 -- system roles that come with every sourcegraph instance
 INSERT INTO
- roles
+ roles (id, name, created_at, deleted_at, readonly)
 VALUES
  (1, 'USER', '2023-01-04 16:29:41.195966+00', NULL, TRUE),
  (2, 'SITE_ADMINISTRATOR', '2023-01-04 16:29:41.195966+00', NULL, TRUE)