I'm having the same issue. After editing the repo file, saving it, and try to reinstall, the file reverts back to before it was edited. Is there another file to edit to stop it from reverting?
I use vi to edit, change the information to update it, the :wq. I then go back into the file to make sure it saved with the changes. I try to install again, and it still tried to download MariaDB 10.4, when I check the repo file, the changes I made are gone.