os: remove non-constant format string in t.Errorf call

Fixes #68744

Change-Id: I12db785fc21c18db69d57cf91e2190b4a3df2484
Reviewed-on: https://go-review.googlesource.com/c/go/+/603515
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/os/os_test.go

@@ -1533,10 +1533,10 @@ func testChtimes(t *testing.T, name string) {
 				t.Log(errormsg)
 				t.Log("Known NetBSD issue (atime not changed on fs mounted with noatime); ignoring.")
 			} else {
-				t.Errorf(errormsg)
+				t.Error(errormsg)
 			}
 		default:
-			t.Errorf(errormsg)
+			t.Error(errormsg)
 		}
 	}