tools: update codecov branch

Codecov is still using the old default branch. Update it to "main", the
current default branch. I'm not sure why it's not picking it up
automatically or if there is a way to ask it to refresh, but setting it
in codecov.yml should resolve the issue.

PR-URL: #47285
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>

codecov.yml

@@ -8,6 +8,7 @@ comment: false
 #  require_changes: true
 
 codecov:
+  branch: main
   notify:
     # Wait for all coverage builds:
     after_n_builds: 2