hydra: remove HYDRA_HOME env var, not used

Dependencies

• [2] YQWH4POV * Simplify.
• [3] JK2QWPH6
• [4] RWIBJ5L4 * Autoflush stdout.
• [5] TULPZ62Y * Perform builds in parallel.
• [6] AFTXA575 * $HYDRA_DATA environment variable.
• [7] T4LLYESZ * Nix expression for building Hydra.
• [8] 7YBYT2LQ
• [9] YTESD75Z * Get rid of zombies.
• [10] 3MDDGGSN * Install the manual.
• [11] DFXIN6YW * Include jquery and tablesorter in the build.

Change contents

• edit in release.nix at line 61
  [2.2883]→[2.2883:2937](∅→∅)
  − --set HYDRA_HOME $out/libexec/hydra \
• edit in src/script/hydra_queue_runner.pl.in at line 14
  [3.266]→[3.1190:1309](∅→∅),[3.1309]→[3.24:25](∅→∅)
  − my $hydraHome = $ENV{"HYDRA_HOME"};

  − die "The HYDRA_HOME environment variable is not set!\n" unless defined $hydraHome;

  −