Substitutes from multiple server

[ludo@gnu.org (Ludovic Courtès)]

It’s now possible to really use multiple substitute servers, as in:

  guix build foo --substitute-urls='http://example.org http://hydra.gnu.org'

Here Guix first looks for substitutes at example.org, and then falls
back to gnu.org for those that were not found.

The basics were in place but commit 55b2fc1 adds the missing bits.

Unfortunately, due to <http://bugs.gnu.org/20217>, the daemon’s
--substitute-urls is overridden by clients, so you really have to pass
--substitute-urls to various ‘guix’ commands.

Unfortunately², because URLs are whitespace-separated, one cannot put
the option in the ‘GUIX_BUILD_OPTIONS’ environment variable, because
that one is simply split up using ‘string-tokenize’.

Anyway, future work!

Ludo’.