fix: Use 4-core runners for test-cargo-unit to resolve disk space issues

The test-cargo-unit job was failing with "No space left on device" errors
during Rust compilation. Switching to ubuntu-latest-4-cores provides:
- More disk space (~14GB vs ~4GB)
- More memory (16GB vs 7GB)
- More CPU cores (4 vs 2)

This targets only the problematic job without affecting other workflows.

Author: celicoo

.github/workflows/build_and_test.yml

@@ -165,6 +165,7 @@ jobs:
       afterBuild: pnpm dlx turbo@${TURBO_VERSION} run test-cargo-unit
       mold: 'yes'
       stepName: 'test-cargo-unit'
+      runs_on_labels: '["ubuntu-latest-4-cores"]'
     secrets: inherit
 
   rust-check: