`guix pull` of an ssh channel fails

[Bone Baboon]

I have a substitute server that needs to build a couple of packages with
patches:

* inetutlis version 2.0 from the core-updates branch instead of 1.9.4
  from the master branch
** https://issues.guix.gnu.org/issue/48214#10

* glib with longer test timeout
** https://issues.guix.gnu.org/issue/48024#12

* offlineimap without tests
** https://issues.guix.gnu.org/issue/48213#2

I am trying to setup a channel for this substitute server's patched Guix
repository. `guix pull` is failing.

On a client of the substitute server I have reconfigured with this in
it's system configuration:

```
    (service guix-service-type
             (guix-configuration
              (authorized-keys
               (list
                (local-file
                 "/path/to/signing-key.pub")))))
```
Where signing-key.pub is the substitute server's public signing key.

I have added the ssh public key of the substitute server to the client
with `ssh-copy-id`.  The substitute server's ssh public key is in
~/.ssh/authorize_keys.  I have also done the reverse with the client's
ssh public key and it is on the substitute server's
~/.ssh/authorize_keys.

This is the contents of ~/.config/guix/channels.scm:

```
(list
 (channel
  (name 'guix)
  (url "ssh://<user>@<ip-address>:22/path/to/patched-guix-repo/")
  (branch "master")))
```

When I run `guix pull` I get this error.

```
Updating channel 'guix' from Git repository at 
'ssh://<user>@<ip-address>:22/path/to/patched-guix-repo/'...
guix pull: error: Git error: error authenticating: no auth sock variable
```

I can successfully run `git clone
ssh://<user>@<ip-address>:22/path/to/patched-guix-repo`.

I can successfully transfer files from the substitute server to the
client with scp without entering a password.

Can a channel use a ssh url?