fix: restrict cross-compilation test to x86_64 runners only

- Only run make run-built-image on ubuntu-latest (x86_64) runners
- ARM64 runners cannot cross-compile for x86_64 due to linker architecture mismatch
- Prevents 'file in wrong format' errors when ARM64 musl-gcc tries to link x86_64 objects

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.github/workflows/ci.yml

@@ -58,7 +58,10 @@ jobs:
     - run: make build
     - run: make test
     - run: make test-e2e
-    - run: make run-built-image
+    # Only run cross-compilation integration test on x86_64 runners
+    - name: Run integration test (cross-compilation)
+      if: matrix.os == 'ubuntu-latest'
+      run: make run-built-image
 
   fmt:
     name: Rustfmt