Don't set cache-read-only for scheduled workflows

For scheduled workflows, we don't know the default branch. In this case cannot determine if
the workflow is running on the default branch, and so cannot know when to enable `cache-read-only`.

Fixes #383

action.yml

@@ -18,7 +18,7 @@ inputs:
       When 'true', existing entries will be read from the cache but no entries will be written.
       By default this value is 'false' for workflows on the GitHub default branch and 'true' for workflows on other branches.
     required: false
-    default: ${{ github.ref_name != github.event.repository.default_branch }}
+    default: ${{ github.event.repository && github.ref_name != github.event.repository.default_branch }}
 
   cache-write-only:
     description: |