chore: downgrade to got@9 in system tests (#1166)

Author: kjin

system-test/package.json

@@ -5,7 +5,7 @@
     "@google-cloud/datastore": "^5.0.0",
     "express": "^4.16.3",
     "google-auto-auth": "^0.10.0",
-    "got": "^10.0.0",
+    "got": "^9.0.0",
     "hard-rejection": "^2.0.0",
     "querystring": "^0.2.0"
   }