use build timestamp as last-modified header on download

Dependencies

• [2] XJFHFZCA * Provide some redirects to build products by type so that we can for
• [*] LBNVQXUB * Build the /build stuff in a separate controller.

Change contents

• edit in src/lib/Hydra/Controller/Build.pm at line 200
  [4.3293]

  [2.774]
  + $c->response->headers->last_modified($c->stash->{build}->timestamp);