cp --sparse=auto doesn't create a sparse file

[AnirbanHalder654322]

The default coreutils cp behaviour uses standard fs copy as fallback when cow is not available in our filesystem.

Steps to reproduce issue

printf hello > a
./target/release/coreutils cp --sparse=auto a b
du -h b
12K    b
du -h --apparent-size b
11K    b

The correct cp behavior

printf hello > a
/usr/bin/cp --sparse=auto a b
du -h b
4.0K    b
du -h --apparent-size b
11K    b```

[AnirbanHalder654322]

Fixed in #6220