bug#63247: guix weather should report on all substitute servers configured

[Steven Roose]

Currently guix weather only reports on the two default guix substitute 
servers. When you have configured other servers in your guix-daemon 
config, I think it would be expected behavior that guix weather also 
checks these for the given package.

Currently using guix weather with an already-configured substitute URL 
requires manually passing that URL in --substitute-urls=.