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

[Leo Prikler]

I think the potential to confuse new users with this enabled by default
is higher than without it.  Assume that Alice just sent a bug report to
Guix, that has been closed or has otherwise been made aware of some bug
affecting a software she uses, for which a fix has been pushed. 
Naturally, she's going to run `guix pull' to test it out, and… not see
any difference to the system she currently had.

I think we do have the obligation to clearly document channels-with-
substitutes-available and make it so that this particular piece of
documentation can easily be found by everyone who is potentially
affected by slow-downs.  Perhaps we might even want to include an
option in the installer to set up /etc/guix/channels.scm to only use
channels with available substitutes.  But using it as a default without
communicating this fact to the users would be a poor idea in my
opinion.

Regards,
Leo