bug#18747: Fixed-output derivations are not substituted

[Ludovic Courtès]

With current ‘master’, fixed-output derivations are not substituted,
even though a substitute is available:

--8<---------------cut here---------------start------------->8---
$ guix build -S acct
The following file will be downloaded:
   /gnu/store/0sszdqmiqmb4mzbgs8h2185fh7p8bfd2-acct-6.6.1.tar.gz
@ build-started 
/gnu/store/b7c2lgbdnr5anzklxkzhjvqxl14n7cqp-acct-6.6.1.tar.gz.drv - 
x86_64-linux 
/var/log/guix/drvs/b7//c2lgbdnr5anzklxkzhjvqxl14n7cqp-acct-6.6.1.tar.gz.drv.bz2
starting download of 
`/gnu/store/0sszdqmiqmb4mzbgs8h2185fh7p8bfd2-acct-6.6.1.tar.gz' from 
`http://ftpmirror.gnu.org/acct/acct-6.6.1.tar.gz'...
following redirection to `http://gnu.cardse.net/acct/acct-6.6.1.tar.gz'...
http://gnu.cardse.net/.../acct-6.6.1.tar.gz     100.0% of 2802.4 KiB
@ build-succeeded 
/gnu/store/b7c2lgbdnr5anzklxkzhjvqxl14n7cqp-acct-6.6.1.tar.gz.drv -
/gnu/store/0sszdqmiqmb4mzbgs8h2185fh7p8bfd2-acct-6.6.1.tar.gz

$ sudo cat /var/guix/substitute-binary/cache/0sszdqmiqmb4mzbgs8h2185fh7p8bfd2
(narinfo (version 1) (cache-uri "http://hydra.gnu.org";) (date 1413472751) 
(value "StorePath:...

$ guix build -S libwebsockets
The following file will be downloaded:
   /gnu/store/1k4c1a6nckhm2hjpprvdrlkxx6yifclw-libwebsockets-1.3
@ build-started 
/gnu/store/8x0jgw0819l7aqi1k1f6i772skmwq1zq-libwebsockets-1.3.drv - 
x86_64-linux 
/var/log/guix/drvs/8x//0jgw0819l7aqi1k1f6i772skmwq1zq-libwebsockets-1.3.drv.bz2
Cloning into '/gnu/store/1k4c1a6nckhm2hjpprvdrlkxx6yifclw-libwebsockets-1.3'...
C-c C-c

$ sudo cat /var/guix/substitute-binary/cache/1k4c1a6nckhm2hjpprvdrlkxx6yifclw
(narinfo (version 1) (cache-uri "http://hydra.gnu.org";) (date 1413466633) 
(value "StorePath:...
--8<---------------cut here---------------end--------------->8---

Here, “The following file will be downloaded” means that a substitute is
found and will be used, as seen also in the cache.  But then we find
ourselves performing the derivation locally regardless.

Ludo’.