hydra: add comment that file is generated by hydra to /etc/nix.machines

Dependencies

• [2] SMCOU72F hydra: add some admin for adding/enabling/etc build machines
• [3] SGNXIOI4 Hydra/32: Add option to force evaluation of a certain jobset via web interface (for admins only)
• [4] QT4FO2HP refactored admin controller, using chains, to avoid using requireadmin on each endpoint
• [*] ZWCTAZGL added newsitems, added some admin options to clear various caches.

Change contents

• replacement in src/lib/Hydra/Controller/Admin.pm at line 13
  [2.62]→[2.62:83](∅→∅)
  − my $result = '';
  [2.62]

  [2.83]
  + my $result = "# GENERATED BY HYDRA\n";