downgrade firebase-tools in rules-unit-testing to be consistent with the rest of the repo (#3646)

Author: Feiyang1

packages/rules-unit-testing/package.json

@@ -30,7 +30,7 @@
     "@google-cloud/firestore": "4.2.0",
     "@types/request": "2.48.5",
     "firebase-admin": "9.0.0",
-    "firebase-tools": "8.7.0",
+    "firebase-tools": "8.6.0",
     "rollup": "2.23.0",
     "rollup-plugin-typescript2": "0.27.1"
   },