Set custom functions directory (#1519)

All our sites use this, but they're inconsistently configured in the web
UI at the moment.

Author: illicitonion

tooling/common-config/netlify.toml

@@ -21,6 +21,7 @@ to = "/.netlify/functions/:splat"
 status = 200
 
 [functions]
+directory = "tooling/netlify/functions"
 external_node_modules = ["node-fetch"]
 
 [[headers]]