os/user: fix formatting of error group lookup message

It was failing like "unknown groupid ᎈ|" instead of "unknown groupid
5000" due to the conversion from int to string.

Updates #14806

Change-Id: I83e4b478ff628ad4053573a9f32b3fadce22e847
Reviewed-on: https://go-review.googlesource.com/20642Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/os/user/lookup_unix.go

@@ -183,7 +183,7 @@ func lookupUnixGid(gid int) (*Group, error) {
 		return nil, fmt.Errorf("user: lookup groupid %d: %v", gid, err)
 	}
 	if result == nil {
-		return nil, UnknownGroupIdError(gid)
+		return nil, UnknownGroupIdError(strconv.Itoa(gid))
 	}
 	return buildGroup(&grp), nil
 }