Swap to net.ErrClosed checks for services

In Go 1.16 `net.ErrClosed` was exported, removing the need to check the
exact text of "use of closed network connection". The stdlib's net listeners
are all setup for this to be a reality, but on Windows containerd uses the
the go-winio projects named pipe implementation as the listener for services.
Before version 0.6.0 this project returned a different error named
`ErrPipeListenerClosed` for using a closed pipe, where this error was just
an `errors.New` with the same text as `net.ErrClosed`, so checking against
`net.ErrClosed` wasn't possible.

Starting in 0.6.0 go-winio has that error assigned to `net.ErrClosed` directly
so this *should* be alright to finally change.

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

cmd/containerd-shim/main_unix.go

@@ -22,6 +22,7 @@ package main
 import (
 	"bytes"
 	"context"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -237,8 +238,7 @@ func serve(ctx context.Context, server *ttrpc.Server, path string) error {
 	logrus.WithField("socket", path).Debug("serving api on unix socket")
 	go func() {
 		defer l.Close()
-		if err := server.Serve(ctx, l); err != nil &&
-			!strings.Contains(err.Error(), "use of closed network connection") {
+		if err := server.Serve(ctx, l); err != nil && !errors.Is(err, net.ErrClosed) {
 			logrus.WithError(err).Fatal("containerd-shim: ttrpc server failure")
 		}
 	}()