try to fix test 'cmdext:TestFailOnStderr'

cmdext/cmdrunner.go

@@ -133,9 +133,6 @@ func run(opt CommandRunner) (CommandResult, error) {
 
 	case <-stderrFailChan:
 		_ = cmd.Process.Kill()
-		for _, lstr := range opt.listener {
-			lstr.Timeout()
-		}
 
 		if fallback, ok := syncext.ReadChannelWithTimeout(outputChan, 32*time.Millisecond); ok {
 			// most of the time the cmd.Process.Kill() should also have finished the pipereader
@@ -160,7 +157,8 @@ func run(opt CommandRunner) (CommandResult, error) {
 		}
 
 	case outobj := <-outputChan:
-		if exiterr, ok := outobj.err.(*exec.ExitError); ok {
+		var exiterr *exec.ExitError
+		if errors.As(outobj.err, &exiterr) {
 			excode := exiterr.ExitCode()
 			for _, lstr := range opt.listener {
 				lstr.Finished(excode)