vendor: roll back golang.org/x/sys to fix compile

Until https://github.com/golang/go/issues/21136 is fixed

vendor/golang.org/x/sys/unix/file_unix.go

@@ -1,27 +0,0 @@
-// Copyright 2017 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package unix
-
-import (
-	"os"
-	"syscall"
-)
-
-// FIXME: unexported function from os
-// syscallMode returns the syscall-specific mode bits from Go's portable mode bits.
-func syscallMode(i os.FileMode) (o uint32) {
-	o |= uint32(i.Perm())
-	if i&os.ModeSetuid != 0 {
-		o |= syscall.S_ISUID
-	}
-	if i&os.ModeSetgid != 0 {
-		o |= syscall.S_ISGID
-	}
-	if i&os.ModeSticky != 0 {
-		o |= syscall.S_ISVTX
-	}
-	// No mapping for Go's ModeTemporary (plan9 only).
-	return
-}