Before this commit, aarch64 users building the following configuration
would end up with an aarch64-linux builder, while after it, they get the
x86_64-linux builder they expect:
```nix
nix.linux-builder = {
enable = true;
package = pkgs.darwin.linux-builder-x86_64;
};
```
Before, in order to get an x86_64-linux builder, they would have needed
to use this configuration instead:
```nix
nix.linux-builder = {
enable = true;
config.nixpkgs.hostPlatform = "x86_64-linux";
systems = ["x86_64-linux"];
};
```
The reason for this is that the linux-builder module calls `override` on
the package option, and the `linux-builder-x86_64` package is also
defined using override:
```nix
linux-builder-x86_64 = linux-builder.override {
modules = [ { nixpkgs.hostPlatform = "x86_64-linux"; } ];
};
```
The module was effectively discarding the `nixpkgs.hostPlatform` option.
Example issue: https://github.com/NixOS/nixpkgs/issues/313784
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.
