Cleaned up linter warnings

src/build/fs/Walk_test.go

@@ -10,10 +10,11 @@ import (
 func TestWalk(t *testing.T) {
 	var files []string
 
-	fs.Walk(".", func(file string) {
+	err := fs.Walk(".", func(file string) {
 		files = append(files, file)
 	})
 
+	assert.Nil(t, err)
 	assert.Contains(t, files, "Walk.go")
 	assert.Contains(t, files, "Walk_test.go")
 }