bug#30143: UX: print warning if substitute server is not authorized

[Ricardo Wurmus]

Suppose I add example.com as a substitute server by passing
“--substitute-urls=https://example.com” to the daemon or the Guix
command line.  I haven’t authorized the signing key, so Guix won’t
accept any of the substitutes from example.com.

Currently, Guix does not make it obvious to the user that a requested
substitute server is ignored because its key is not authorized.  We
should print a clear warning in this case.

(guix scripts authenticate) already includes “validate-signature”, which
aborts with an error if the key is not authorized, but we don’t seem to
use it.

--
Ricardo