test-packaging.yaml: Omit prerequisite step for jw-pkg

Don't build and install jw-pkg official before a jw-pkg test build. jw-pkg should be self-contained enough to be able to build itself.
Signed-off-by: Jan Lindemann <jan@janware.com>

.github/workflows/test-packaging.yaml

@@ -30,6 +30,7 @@ jobs:
 
       - name: Install build prerequisites
         shell: bash
+        if: ${{ github.event.repository.name != 'jw-pkg' }}
         run: |
           set -euxo pipefail
           export PROJECTS_DIR_REMOTE_BASE=${{ github.server_url }}