source-postgres: initdb tweaks to make CI work

For some reason the test suite works fine locally when running
against the dockerized test Postgres instance, but when running
in a CI build it fails because when user `flow_capture` tries to
create the `public.flow_watermarks` table they get a permission
denied error.

Which is doubly weird because:

  1. A newly created user should by default have permissions to
     interact with the `public` schema and create tables there.
  2. It works when running locally, and both local and CI builds
     are using the same `source-postgres/docker-compose.yaml` and
     just running `docker compose up` on that, so they should have
     identical behavior.

So let's see if explicitly granting `flow_capture` the `CREATE`
and `USAGE` permissions on schema `public` fixes the inconsistency.

source-postgres/docker-entrypoint-initdb.d/init-user-db.sh

@@ -9,5 +9,7 @@ psql -v ON_ERROR_STOP=1 --username "$POSTGRES_USER" --dbname "$POSTGRES_DB" <<-E
     ALTER DEFAULT PRIVILEGES IN SCHEMA test GRANT SELECT ON TABLES to flow_capture;
     GRANT SELECT ON ALL TABLES IN SCHEMA test TO flow_capture;
 
+    GRANT CREATE, USAGE ON SCHEMA public TO flow_capture;
+
     CREATE PUBLICATION flow_publication FOR ALL TABLES;
 EOSQL