Experimental network.buildShell

[srhb]

Basic testing done.

This grants users a medium-to-large escape hatch in that they may define network.buildShell to be a shell-compatible drv from which morph can and will execute nix-build.

One could, for instance, override the nix-build that's on $PATH by using this. Or just have funny messages displayed whenever a shell is opened.

    network.buildShell = pkgs.mkShell {
      name = "test";
      shellHook = ''
        echo "Woo, here we go!"
      '';
    };

[srhb]

If I had more confidence in this, I would entirely drop the possibility of not running nix-build (or anything but eval) from inside a nix-shell, but I'm not quite there yet. In general I feel like inverting control quite a bit more towards the Nix side is the sane thing to do.

[srhb]

There's an issue with hostnames with special characters because of the way quoting differs in the two methods. Need to figure out a way to fix that.

[johanot]

LGTM other than the single comment about the debug print. Also, please give all files a go fmt.

[johanot]

Added go fmt.