unix: remove the use of ioutil

Trivial changes.

Change-Id: If7ffa47bb4c3b2bae784091e94516b0f4fc197d2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/526300
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>

unix/mkpost.go

@@ -16,7 +16,7 @@ import (
 	"bytes"
 	"fmt"
 	"go/format"
-	"io/ioutil"
+	"io"
 	"log"
 	"os"
 	"regexp"
@@ -41,7 +41,7 @@ func main() {
 		}
 	}
 
-	b, err := ioutil.ReadAll(os.Stdin)
+	b, err := io.ReadAll(os.Stdin)
 	if err != nil {
 		log.Fatal(err)
 	}