[exec] Replace syscall with /x/sys/unix

This replaces the syscall usage with sys/unix in the execution code

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

linux/shim/init.go

@@ -10,6 +10,8 @@ import (
 	"sync"
 	"syscall"
 
+	"golang.org/x/sys/unix"
+
 	"github.com/containerd/containerd"
 	shimapi "github.com/containerd/containerd/api/services/shim"
 	"github.com/crosbymichael/console"
@@ -175,7 +177,7 @@ func (p *initProcess) killAll(context context.Context) error {
 }
 
 func (p *initProcess) Signal(sig int) error {
-	return syscall.Kill(p.pid, syscall.Signal(sig))
+	return unix.Kill(p.pid, syscall.Signal(sig))
 }
 
 func (p *initProcess) Stdin() io.Closer {