Fixed checkout depth error in scheduled weekly CI workflow

[Arlodotexe]

After testing #696 in https://github.com/CommunityToolkit/Labs-Windows/actions/runs/16616965286/job/47011751224, we found the following error:

could not compute title or body defaults: failed to run git: fatal: ambiguous argument 'origin/rel/weekly...main': unknown revision or path not in the working tree.

To fix this issue, fetch-depth: 0 was added to the checkout step so that all branches are pulled.

This will allow the --fill-verbose parameter to locally diff the two branches and compute the commits that were changed between weekly releases.