ci: fix by expanding env var CHANGED_SUBDIRS (#62)

Author: felix-seifert

.github/workflows/kubernetes_apply_dispatch.yaml

@@ -42,4 +42,4 @@ jobs:
           token: ${{ secrets.PAT_FOR_TRIGGERING_PRIVATE_WORKFLOWS }}
           repository: felix-seifert/gohfert-cluster-private-runners
           event-type: kubernetes-manifests-apply
-          client-payload: '{"ref": "${{ github.ref }}", "changed_kubernetes_subdirs": "$CHANGED_SUBDIRS"}'
+          client-payload: '{"ref": "${{ github.ref }}", "changed_kubernetes_subdirs": "${{ env.CHANGED_SUBDIRS }}"}'