Hi!

I've been using channel-with-substitutes-available recently, and
noticed that at one point it's returned a commit from the staging
branch instead of master.  On further investigation, it seems the API
needs a jobset parameter to restrict the results to only main guix
builds.

eg. http://ci.guix.gnu.org/api/latestbuilds?nr=5&job=guix.x86_64-linux
vs. http://ci.guix.gnu.org/api/latestbuilds?nr=5&job=guix.x86_64-linux&jobset=guix

Hopefully the above shows the problem, the first link includes builds
with jobset "staging" which probably shouldn't be returned as a
suitable guix revision for guix pull.

This patch adds a jobset parameter to restrict the substitute search.

Cheers,
Andrew