bug#45302: Avahi substitute discovery keeps trying to ping unaccessible servers

[Pierre Neidhardt <mail@ambrevar.xyz>]

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I've set up my desktop and laptop to use the new substitute discovery
feature, it's awesome!

However, when I put my desktop to sleep and run a Guix command on my
laptop that requires access to a sbustitute server, I see this:

--8<---------------cut here---------------start------------->8---
$ guix build ncdu
substitute: guix substitute: warning: 10.0.0.5: connection failed: No route to host
substitute: updating substitutes from 'http://ci.guix.gnu.org'... 100.0%
0.0 MB will be downloaded:
   /gnu/store/p70r4maqgh6ghl25h5a99w7sf1jidap8-ncdu-1.15.1
substituting /gnu/store/p70r4maqgh6ghl25h5a99w7sf1jidap8-ncdu-1.15.1...
--8<---------------cut here---------------end--------------->8---

The warning

--8<---------------cut here---------------start------------->8---
substitute: guix substitute: warning: 10.0.0.5: connection failed: No route to host
--8<---------------cut here---------------end--------------->8---

pops up on every download, which adds some 2s delay each time.  This
makes the whole process much slower.

For your information, Avahi does not find my desktop:

--8<---------------cut here---------------start------------->8---
$ sudo avahi-browse -al
Password: 
+ wlp2s0 IPv6 FOO___s MacBook Pro                        _companion-link._tcp local
+ wlp2s0 IPv4 FOO___s MacBook Pro                        _companion-link._tcp local
  C-c C-cGot SIGINT, quitting.
--8<---------------cut here---------------end--------------->8---

-- 
Pierre Neidhardt
https://ambrevar.xyz/

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