build.sh: set creation is serialized

[Andrew Doran <ad%netbsd.org@localhost>]

In src/src/distrib/sets/Makefile:

---
revision 1.56
date: 2007/03/10 20:34:57;  author: apb;  state: Exp;  lines: +5 -2
branches:  1.56.4;  1.56.10;  1.56.12;
Serialise making the "sets" tarballs in the maketars target.
On many systems, making them in parallel causes very poor
system performance due to disk I/O.  (Requested by jmmv.)
---

On my development system this adds a lot of time to a release build, and I'd
like to revert it. My system is not affected, but if it were I don't see
why any of these would not be a valid solution:

o use smaller value for -j
o analyze the problem on the affected systems
o buy more spindles

Thanks,
Andrew