[bug#43516] [PATCH core-updates] packages: Enable multi-threaded xz compression when repacking source.

[Leo Famulari]

On Sat, Sep 19, 2020 at 01:03:57PM -0400, Maxim Cournoyer wrote:
> The xz compression is slow; using multiple threads/cores yields a linear
> performance improvement.
> 
> * guix/packages.scm (patch-and-repack): Ensure xz is invoked with --threads=N
> by setting the XZ_DEFAULTS environment variable.

We tried this previous but reverted it because the archives were not
bit-reproducible:

https://git.savannah.gnu.org/cgit/guix.git/commit/?id=3e95125e9bd0676d4a9add9105217ad3eaef3ff0

It's really a shame... it would be nice to reduce the time used for XZ
compression. But the bandwidth used to move the results is even more
expensive in terms of time and money, since most people should get
substitutes.