Unable to sign in (link) for existing users when using anonymous

[eifr]

Bug report

Describe the bug

When using Anonymous sign-in, a user that already signed in before is unable to sign in again and I'm getting
Identity+is+already+linked+to+another+user` instead.

To Reproduce

1. enable anonymous sign in (for me its automatically anon signing in when not authenticated
2. sign in with oAuth
3. sign out
4. sign in again

Expected behavior

The new anon user should be merged with the existing user

Additional context

probably related to supabase/auth#1525