fix: add retry logic for flaky cache.nixos.org

Build can fail with EOF errors when fetching from cache.
Add fallback to build from source if cache fails.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.forgejo/workflows/build-image.yml

@@ -16,9 +16,16 @@ jobs:
 
       - name: Build OCI image
         run: |
+          # Try with cache first, fall back to building from source
           nix build .#solidhaus-image \
             --print-build-logs \
-            --show-trace
+            --show-trace || {
+              echo "Build failed, retrying with --no-substitute to build from source..."
+              nix build .#solidhaus-image \
+                --print-build-logs \
+                --show-trace \
+                --option substitute false
+            }
 
       - name: Push to registry
         if: github.ref == 'refs/heads/main'