ci: {{ pr-number }} should be {{ inputs.pr-number }}

One of the joys to rely on the `pull_request_target` is to only be able
to catch those after those are merged.

Fixes: #7595

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

.github/workflows/ci.yaml

@@ -48,7 +48,7 @@ jobs:
       - name: Docker build and push
         uses: docker/build-and-push-action@v3
         with:
-          tags: ghcr.io/kata-containers/test-images:unencrypted-${{ pr-number }}
+          tags: ghcr.io/kata-containers/test-images:unencrypted-${{ inputs.pr-number }}
           push: true
           context: .
           platforms: linux/amd64, linux/s390x