bug#26608: channels.scm supporting substitutes

[Nome Grey]

Ludovic posted some channels.scm code in September 2018 supporting using more substitutes.  Unfortunately his code no longer functions due to an upgrade of guile-json in guix.

I've tried to learn enough guile to upgrade the code to the newer json structures, and posted my changes to github at https://github.com/nomr72/guix-substitutes-channel .  It seems to run again now.

Maybe I can learn enough to upgrade it to check the 'guix-master' evaluations to find the latest build of key packages.  We'll see.