Wrap %default-guix-channel inside channel-with-substitutes-available by default?

[Mekeor Melire]

I think, by default, the focus should be on speeding up Guix (and
specifically `guix pull`). I suggest to wrap %default-guix-channel
inside channel-with-substitutes-available, like this:

(define %default-channels
  (list (channel-with-substitutes-available
    %default-guix-channel
    "https://ci.guix.gnu.org";)))

This way, beginners would enjoy a little speed-up, and sophisticated
users could still use (list %default-guix-channel).

What do you think?