Read the map keys from GOOGLE_MAPS_KEY and MAPBOX_KEY instead of the source (#3092)

preview/.env.example

@@ -0,0 +1,7 @@
+# Google Maps JavaScript API key for the map demo page (map-fullsize).
+# Optional. Restrict the key by HTTP referrer in the Google Cloud console.
+GOOGLE_MAPS_KEY=
+
+# Mapbox public access token (pk.…) for the Mapbox GL map demos.
+# Optional. Restrict the token by URL in the Mapbox account settings.
+MAPBOX_KEY=