make JSON writing optional

[zimbatm]

allow to pass writeJSON = false; and get the raw data instead.

This is useful if one wants to avoid writing secrets in the /nix/store. It also speeds up the rendering quite a bit.

Eg:

./render.sh

#!/usr/bin/env bash
set -euo pipefail

imageTag=foobar

toYAML() {
  ruby -ryaml -rjson -e 'puts YAML.dump(JSON.load(ARGF))'
}

nix-instantiate ./default.nix \
  --eval --strict --json \
  --argstr imageTag "$imageTag" \
  | toYAML 

default.nix

{ imageTag ? "latest" }:
let
  # where kubenix is added as an overlay
  pkgs = import ../nix {};

  configuration = import ./configuration.nix { inherit imageTag; };

  manifest = pkgs.kubenix.buildResources {
    inherit configuration;
    writeJSON = false;
  };
in
  manifest