reduce parallelism for cross tests.

Maybe it helps with the memory allocation failures

Author: rklaehn

.github/workflows/ci.yaml

@@ -143,7 +143,7 @@ jobs:
     - uses: taiki-e/install-action@cross
 
     - name: test
-      run: cross test --all --target ${{ matrix.target }} -- --test-threads=12
+      run: cross test --all --target ${{ matrix.target }} -- --test-threads=4
       env:
         RUST_LOG: ${{ runner.debug && 'TRACE' || 'DEBUG' }}