This commit adds a protocol option for the `linux-builder` and defaults
it to `ssh-ng`. I have observed it needing this with the following:
``` sh
$ nix store ping --store ssh://linux-builder
Store URL: ssh://linux-builder
$ nix store ping --store ssh-ng://linux-builder
Store URL: ssh-ng://linux-builder
Version: 2.18.1
Trusted: 0
```
This seems to make the difference on whether or not Nix picks up
`linux-builder` as an available builder.
61c5879b5a