fix: use if statement instead of continue-on-error for Attic

docker-build-nix/action.yaml

@@ -64,7 +64,6 @@ runs:
     - name: Push build artifacts to Attic cache
       shell: bash
       if: env.ATTIC_TOKEN != ''
-      continue-on-error: true
       env:
         ATTIC_TOKEN: ${{ env.ATTIC_TOKEN }}
       run: |
@@ -77,7 +76,9 @@ runs:
         EOF
 
         # Push entire closure to cache (don't fail if this fails)
-        attic push "${{ inputs.cache-name }}" ./result || echo "Warning: Failed to push to Attic cache, continuing anyway"
+        if ! attic push "${{ inputs.cache-name }}" ./result; then
+          echo "Warning: Failed to push to Attic cache, continuing anyway"
+        fi
 
     - name: Load image into Docker
       shell: bash