DEV: Run yarn install when running bin/ember-cli (#13102)

Some people have noticed that if we change the packages in package.json
that they have to manually run `yarn install` or Discourse won't work.

This adds `yarn install` to the `bin/ember-cli` helper we run. It seems
quite fast if there is nothing to install so it shouldn't hurt to do
this every time we start the server.

bin/ember-cli

@@ -43,4 +43,5 @@ if !args.include?("--proxy")
   args << PROXY
 end
 
+system "yarn install --cwd #{yarn_dir}"
 exec "yarn", *args.to_a.flatten