builder/copy-unix: fix filepath.Walk args

Mark unused WalkFunc arguments as such, to get rid of the
following linterwarning:

> builder/dockerfile/copy_unix.go:28:71: SA4009: argument err is overwritten before first use (staticcheck)
> 	return filepath.Walk(source, func(fullpath string, info os.FileInfo, err error) error {

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

builder/dockerfile/copy_unix.go

@@ -25,7 +25,7 @@ func fixPermissions(source, destination string, identity idtools.Identity, overr
 
 	// We Walk on the source rather than on the destination because we don't
 	// want to change permissions on things we haven't created or modified.
-	return filepath.Walk(source, func(fullpath string, info os.FileInfo, err error) error {
+	return filepath.Walk(source, func(fullpath string, _ os.FileInfo, _ error) error {
 		// Do not alter the walk root iff. it existed before, as it doesn't fall under
 		// the domain of "things we should chown".
 		if skipChownRoot && source == fullpath {