…dMachines.*.systems` option

…tem`

This commit passes more options to  nix.buildMachines. Namely the mandatoryFeatures and speedFactor settings have been missing.

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.

…builder-big-config-take-2

…o linux-builder-big-config-take-2