Change linux runtime(w shim) to builtin

Because of the plugin findings and having the default runtime builtin
this makes it much better for development and testing.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/containerd/builtins.go

@@ -2,6 +2,7 @@ package main
 
 // register containerd builtins here
 import (
+	_ "github.com/docker/containerd/linux"
 	_ "github.com/docker/containerd/services/content"
 	_ "github.com/docker/containerd/services/execution"
 )