Assign ErrPipeListenerClosed to net.ErrClosed

This error used to just be a `errors.New` with the text exactly matching
the "use of closed network connection" that you'd get from the Go stdlib.
Now that that error was exported as `net.ErrClosed` in Go 1.16 we should
be able to swap to this safely, and anyone who relied on checking the
string explicitly should still be fine.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

pipe.go

@@ -89,8 +89,7 @@ const (
 
 var (
 	// ErrPipeListenerClosed is returned for pipe operations on listeners that have been closed.
-	// This error should match net.errClosing since docker takes a dependency on its text.
-	ErrPipeListenerClosed = errors.New("use of closed network connection")
+	ErrPipeListenerClosed = net.ErrClosed
 
 	errPipeWriteClosed = errors.New("pipe has been closed for write")
 )