libct: use errors.AsType

Simplify error type inspection. For the benefits, see
e.g. https://antonz.org/accepted/errors-astype/.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/container_linux.go

@@ -1214,8 +1214,7 @@ func ignoreTerminateErrors(err error) error {
 	// The (*Cmd).Wait documentation says: "If the command fails to run
 	// or doesn't complete successfully, the error is of type *ExitError".
 	// Filter out such errors (like "exit status 1" or "signal: killed").
-	var exitErr *exec.ExitError
-	if errors.As(err, &exitErr) {
+	if _, ok := errors.AsType[*exec.ExitError](err); ok {
 		return nil
 	}
 	if errors.Is(err, os.ErrProcessDone) {

libcontainer/restored_process.go

@@ -49,8 +49,7 @@ func (p *restoredProcess) wait() (*os.ProcessState, error) {
 	// maybe use --exec-cmd in criu
 	err := p.cmd.Wait()
 	if err != nil {
-		var exitErr *exec.ExitError
-		if !errors.As(err, &exitErr) {
+		if _, ok := errors.AsType[*exec.ExitError](err); !ok {
 			return nil, err
 		}
 	}

libcontainer/state_linux_test.go

@@ -42,8 +42,7 @@ func testTransitions(t *testing.T, initialState containerState, valid []containe
 			if err == nil {
 				t.Fatal("transition should fail")
 			}
-			var stErr *stateTransitionError
-			if !errors.As(err, &stErr) {
+			if _, ok := errors.AsType[*stateTransitionError](err); !ok {
 				t.Fatal("expected stateTransitionError")
 			}
 		})