Add some more fields to the JSON output of /build

Dependencies

• [2] LZVO64YG Merge in the first bits of the API work
• [*] LBNVQXUB * Build the /build stuff in a separate controller.

Change contents

• edit in src/lib/Hydra/Controller/Build.pm at line 114
  [2.4684]

  [2.4684]
  + 'nixname',

  + 'description',

  + 'drvpath',

  + 'system',