[Darwin] clonefile() fails if the target already exists

This breaks the expected `copyFile` semantics, which should truncate an existing file instead of throwing an error.

We could pro-actively remove the target:

```diff
func copyFile(target, source string) error {
+ if err := os.Remove(target); err != nil {
+ if !errors.Is(err, unix.ENOENT) {
+ return fmt.Errorf("removing copyFile target failed: %w", err)
+ }
+ }
if err := unix.Clonefile(source, target, unix.CLONE_NOFOLLOW); err != nil {
if !errors.Is(err, unix.ENOTSUP) {
return fmt.Errorf("clonefile failed: %w", err)
```

But I'm not sure if this could become a problem when `clonefile` is not supported, or when the copy operation is cross-device, and we still fall back to `copyFile`.

I'm happy to create a PR with the patch above, but would like confirmation first that this would indeed be the correct approach.