bug#71133: linux-libre-guix.tar.xz CI times out on aarch64-linux

[Richard Sent]

Hi Guix!

On aarch64 platforms, the linux-libre-guix.tar.xz.drv build times out
[1].

This package is known to have a long build time [2].

Users can work around this by running the build locally with a (very,
very large) max-silent-time (~28800 seconds on my machine). Given the
impact of this issue (unable to build linux-libre on 64-bit Arm without
running a multi-hour build), I think it's worth revisiting if the
substitute server timeout should be increased again.

Alternatively, perhaps we could fetch the officially released
Linux-libre tarballs instead of computing them ourselves [3].

[1]: https://ci.guix.gnu.org/build/4711550/log/raw
[2]: https://mail.gnu.org/archive/html/guix-devel/2021-08/msg00077.html
[3]: http://linux-libre.fsfla.org/pub/linux-libre/releases/

CC'ing Christopher Baines since this deals with substitutes.

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.