guix copy connection problems

[Jack Hill]

Hello Guix,

I'm trying to use `guix copy` to copy some store items from a more 
powerful computer to my laptop. I've tried to set everything up correctly 
with ssh-agent, and I can ssh without being prompted for credentials, but 
guix-initiated connections are timing out, so clearly I'm missing 
something, but I don't know what. Perhaps the following session transcript 
with show what I'm missing:

```
$ env|grep -i ssh
SSH_AUTH_SOCK=/run/user/1000/keyring/ssh
jackhill@alperton ~$ ssh-add -l
256 SHA256:g7qjlV99AHx3R2/ZlkeMGPqnYGzwPJjhVY5y7WX1a/E jackhill@alperton 
(ED25519)
jackhill@alperton ~$ ssh jackhill@remote
Last login: Fri Jul 26 21:38:23 2019 from 107.15.18.231
jackhill@remote ~$ logout
Connection to remote closed.
jackhill@alperton ~$ guix copy --from="jackhill@remote" icecat
guix copy: error: SSH connection to 'remote' failed: Timeout connecting to 
remote
```

Best,
Jack