local: fix cleanRootPath on Windows after go1.21.4 stdlib update

Similar to
acf1e2df84,
go1.21.4 appears to have broken sync.MoveDir on Windows because
filepath.VolumeName() returns `\\?` instead of `\\?\C:` in cleanRootPath. It
looks like the Go team is aware of the issue and planning a fix, so this may
only be needed temporarily.

backend/local/local.go

@@ -1447,6 +1447,10 @@ func cleanRootPath(s string, noUNC bool, enc encoder.MultiEncoder) string {
 	if runtime.GOOS == "windows" {
 		s = filepath.ToSlash(s)
 		vol := filepath.VolumeName(s)
+		if vol == `\\?` && len(s) >= 6 {
+			// `\\?\C:`
+			vol = s[:6]
+		}
 		s = vol + enc.FromStandardPath(s[len(vol):])
 		s = filepath.FromSlash(s)
 		if !noUNC {

fs/sync/sync_test.go

@@ -1351,6 +1351,22 @@ func testServerSideMove(ctx context.Context, t *testing.T, r *fstest.Run, withFi
 	}
 }
 
+// Test MoveDir on Local
+func TestServerSideMoveLocal(t *testing.T) {
+	ctx := context.Background()
+	r := fstest.NewRun(t)
+	f1 := r.WriteFile("dir1/file1.txt", "hello", t1)
+	f2 := r.WriteFile("dir2/file2.txt", "hello again", t2)
+	r.CheckLocalItems(t, f1, f2)
+
+	dir1, err := fs.NewFs(ctx, r.Flocal.Root()+"/dir1")
+	require.NoError(t, err)
+	dir2, err := fs.NewFs(ctx, r.Flocal.Root()+"/dir2")
+	require.NoError(t, err)
+	err = MoveDir(ctx, dir2, dir1, false, false)
+	require.NoError(t, err)
+}
+
 // Test move
 func TestMoveWithDeleteEmptySrcDirs(t *testing.T) {
 	ctx := context.Background()