internal/jsonrpc2_v2: update for go1.16+

Change-Id: If70bb4b7ab236ae921ff23d123cb38d476cfa665
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676516
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/jsonrpc2/net.go

@@ -100,14 +100,14 @@ func (l *netPiper) Accept(context.Context) (io.ReadWriteCloser, error) {
 	// preferring the latter if already closed at the start of Accept.
 	select {
 	case <-l.done:
-		return nil, errClosed
+		return nil, net.ErrClosed
 	default:
 	}
 	select {
 	case rwc := <-l.dialed:
 		return rwc, nil
 	case <-l.done:
-		return nil, errClosed
+		return nil, net.ErrClosed
 	}
 }
 
@@ -133,6 +133,6 @@ func (l *netPiper) Dial(ctx context.Context) (io.ReadWriteCloser, error) {
 	case <-l.done:
 		client.Close()
 		server.Close()
-		return nil, errClosed
+		return nil, net.ErrClosed
 	}
 }