Get backport workflow to work in NuGet repos (#6857)

Author: zivkan

.github/workflows/backport.yml

@@ -13,7 +13,8 @@ jobs:
   backport:
     uses: dotnet/arcade/.github/workflows/backport-base.yml@main
     with:
+        repository_owners: 'NuGet'
         pr_description_template: |
           Backport of #%source_pr_number% to %target_branch%
 
-          /cc %cc_users%
+          /cc %cc_users%