Fix ignored --projectRoot/watchFolders arguments (#21165)

Summary:
Due to Kureev's cleanup in facebook/react-native@c4a66a8 some arguments are no longer properly passed to metro, but just the default values. This broke the `--projectRoot` and `--watchFolders` arguments used by storybook to change the bundle entrypoint.

This PR simply fixes the regression by assigning these values from argv instead of reading config defaults.

Related: the undocumented `REACT_NATIVE_APP_ROOT` env var is broken, not sure for how long, but I can fix it as a part of this PR or make a new one.
Pull Request resolved: facebook/react-native#21165

Differential Revision: D9929642

Pulled By: hramos

fbshipit-source-id: 00485a0429bd4301de03e5cb455e928878c4ba8f

server/runServer.js

@@ -58,6 +58,8 @@ async function runServer(args: Args, config: ConfigT) {
   config.server.port = args.port;
   config.reporter = reporter;
   config.resetCache = args.resetCache;
+  config.projectRoot = args.projectRoot;
+  config.watchFolders = args.watchFolders.slice(0);
   config.server.enhanceMiddleware = middleware =>
     middlewareManager.getConnectInstance().use(middleware);