Only check locally when deduplicating usernames (#13581)

When deduplicating account usernames for OAuthable users, the routine did check if any account was known with that username, including remote accounts. This caused some unnecessary deduplication, and usernames ending with unexpected trailing _1.
This fixes #13580

app/models/concerns/omniauthable.rb

@@ -82,7 +82,7 @@ module Omniauthable
       username = starting_username
       i        = 0
 
-      while Account.exists?(username: username)
+      while Account.exists?(username: username, domain: nil)
         i       += 1
         username = "#{starting_username}_#{i}"
       end