fix the nightly image tags

Author: netroy

.github/workflows/docker-images-nightly.yml

@@ -48,5 +48,5 @@ jobs:
           cache-from: type=gha
           cache-to: type=gha,mode=max
           tags: |
-            ghcr.io/${{ github.repository_owner }}/n8n:${{ env.N8N_TAG }}
-            ${{ secrets.DOCKER_USERNAME }}/n8n:${{ env.N8N_TAG }}
+            ghcr.io/${{ github.repository_owner }}/n8n:nightly
+            ${{ secrets.DOCKER_USERNAME }}/n8n:nightly