fix(ci): Don't sign kernel on PR (ublue-os#590)

Signed-off-by: RJ Sampson <[email protected]>

.github/workflows/reusable-build.yml

@@ -188,6 +188,7 @@ jobs:
 
       - name: Sign kernel
         uses: ublue-os/[email protected]
+        if: github.event_name != 'pull_request'
         with:
           image: ${{ steps.build_image.outputs.image }}
           default-tag: ${{ env.DEFAULT_TAG }}