ci: add the environment variable for testing pdo-pgsql on github

Author: KentarouTakeda

.github/workflows/push.yml

@@ -61,6 +61,7 @@ jobs:
       MYSQL_TEST_HOST: mysql
       PDO_MYSQL_TEST_DSN: mysql:host=mysql;dbname=test
       PDO_MYSQL_TEST_HOST: mysql
+      PDO_PGSQL_TEST_DSN: pgsql:host=postgres port=5432 dbname=test user=postgres password=postgres
     strategy:
       fail-fast: false
       matrix: