all: simplify code by using modern Go constructs

Generated using modernize by running:

    go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...

Change-Id: Ifc7d61cf6735cc53f2bdf890a338961f55075af5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/661975
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

unix/internal/mkmerge/mkmerge_test.go

@@ -498,7 +498,7 @@ func diffLines(t *testing.T, got, expected []byte) {
 func addLineNr(src []byte) []byte {
 	lines := bytes.Split(src, []byte("\n"))
 	for i, line := range lines {
-		lines[i] = []byte(fmt.Sprintf("%d: %s", i+1, line))
+		lines[i] = fmt.Appendf(nil, "%d: %s", i+1, line)
 	}
 	return bytes.Join(lines, []byte("\n"))
 }