interp: don't test cancelling a read via os.Stdin

Now that we use file deadlines directly, the test correctly reads
(0, io.EOF) from stdin because it is empty.
Use an open pipe, which should correctly wait for input forever.

Note that this fails on GOOS=windows for me on Wine
because it seems like os.Pipe files do not support deadlines there,
even though the docs say that OS pipes should generally work:

    On most systems ordinary files do not support deadlines, but pipes do.

Let's see what CI with real Windows thinks.
We can then adjust GOOS=windows on Wine accordingly.

interp/interp_test.go

@@ -4039,22 +4039,14 @@ func TestCancelreader(t *testing.T) {
 	// timeout.
 	defer cancel()
 
-	var stdinRead *os.File
-	if runtime.GOOS == "windows" {
-		// On Windows, the cancelreader only works on stdin
-		stdinRead = os.Stdin
-	} else {
-		var stdinWrite *os.File
-		var err error
-		stdinRead, stdinWrite, err = os.Pipe()
-		if err != nil {
-			t.Fatalf("Error calling os.Pipe: %v", err)
-		}
-		defer func() {
-			stdinWrite.Close()
-			stdinRead.Close()
-		}()
+	stdinRead, stdinWrite, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("Error calling os.Pipe: %v", err)
 	}
+	defer func() {
+		stdinWrite.Close()
+		stdinRead.Close()
+	}()
 	r, _ := interp.New(interp.StdIO(stdinRead, nil, nil))
 	now := time.Now()
 	errChan := make(chan error)