bug#18747: Fixed-output derivations are not substituted

[Ludovic Courtès]

I just realized that this is fixed by just removing #:local-build? #t
from (guix download).

Initially #:local-build? (aka. ‘preferLocalBuild’) meant “avoid
offloading this derivation”, but Nix commit 55586527 (June 2013) changed
it to “avoid offloading *or substituting* this derivation”, which I
somehow overlooked for all this time.

I think we to distinguish between whether to offload and whether to
substitute.  Typically, we don’t want to offload downloads, because
that’s inefficient.  However, we may want to substitute downloads,
because then if the upstream download URL has become invalid, we can
still get the substitute from hydra.gnu.org.

Until then, I think I’ll just comment out #:local-build? #t in (guix
download) and friends.

Thoughts?

Thanks,
Ludo’.