Hydra/62 fix, broken nar download

Dependencies

• [2] W3HI52QV use NIX_*_DIR env vars when defined
• [3] NYC3LZKH * Generate NAR archives on the fly. Necessary for producing channels
• [4] J5UVLXOK * Start of a basic Catalyst web interface.
• [5] MVB7RRLT * Move NARs from the NixChannel controller to the Root controller and
• [6] 5NO7NCKT * Refactoring.
• [7] SMM4HQTP * Put actions related to builds under /build (e.g. /log/<buildid>

Change contents

• replacement in src/lib/Hydra/Controller/Root.pm at line 191
  [3.2637]→[2.400:467](∅→∅)
  − $path = ($ENV{NIX_STORE_DIR}."/$path" || "/nix/store/$path" );
  [3.2637]

  [3.2637]
  + $path = ($ENV{NIX_STORE_DIR} || "/nix/store")."/$path";