work around need for a local runtimeconfig (ish)

This still scares me because if you ever accidentally invoke firebase
shell directly rather than via npm, you could use the wrong config, but
it's better than nothing.

From firebase/firebase-tools#477 (comment)

src/firebase_functions/functions/package.json

@@ -3,7 +3,7 @@
   "description": "Cloud Functions for Firebase",
   "scripts": {
     "serve": "firebase serve --only functions",
-    "shell": "firebase functions:shell",
+    "shell": "firebase functions:config:get > .runtimeconfig.json && firebase functions:shell",
     "start": "npm run shell",
     "deploy": "firebase deploy --only functions",
     "logs": "firebase functions:log"