get_cert doesn't work with SSH agents

In recent versions of gpg, it [can serve as an SSH agent](https://wiki.archlinux.org/index.php/GnuPG#SSH_agent). This allows you to easily store your SSH key on a hardware device, like a yubikey.

Unfortunately, in this setup, if you're using ssh-ca, `get_cert` will fail with:

```
$ get_cert ""
Unable to find private key matching certificate.
```

Because it tries to find the private key in ~/.ssh. Since the private key is stored on a hardware device, it doesn't find it.

A simple workaround is to just `curl "" ~/.ssh/id_rsa-cert.pub` instead of using `get_cert`, but it would be nice if `get_cert` just worked.