go.mod: pin github.com/tailscale/mkctr (try #2)

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tstest/tools/tools.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file exists just so `go mod tidy` won't remove
+// tool modules from our go.mod.
+
+//go:build tools
+// +build tools
+
+package tools
+
+import (
+	_ "github.com/tailscale/mkctr"
+	_ "honnef.co/go/tools/cmd/staticcheck"
+)