Use older package for ErrNotExist

Although it comes from io/fs today, its original source was os.

Author: rkennedy

sh/cmd_test.go

@@ -3,7 +3,6 @@ package sh
 import (
 	"bytes"
 	"errors"
-	"io/fs"
 	"os"
 	"testing"
 )
@@ -78,7 +77,7 @@ func TestWrappedError(t *testing.T) {
 	if err == nil {
 		t.Fatalf("Expected to fail running %s", os.Args[0]+"-doesnotexist")
 	}
-	if !errors.Is(err, fs.ErrNotExist) {
+	if !errors.Is(err, os.ErrNotExist) {
 		t.Fatalf("Expected error to be like ErrNotExist, got %#v", err)
 	}
 }