Use stable Nix

Dependencies

• [2] KNLKTCDM Use pkgconfig to find Nix
• [3] R6O64PAE Use a nicer version string
• [4] UHMUHQMU hydra: fix tarball build, add pre suffix to tarballs
• [5] 6NWLMLSS remove nix from buildinputs
• [6] 4C5F6SZV add openssl to deps
• [7] LOHWNXEJ Cleanup
• [8] 6K5PBUUN Use buildEnv to combine Hydra's Perl dependencies
• [9] T4LLYESZ * Nix expression for building Hydra.
• [10] V2ALOFRR Fix the test
• [11] WYM574OI add nix to buildinputs
• [12] M3A5PZIH hydra: Clarify the dependency on BDW-GC.

Change contents

• replacement in release.nix at line 40
  [3.60]→[2.57:146](∅→∅)
  − [ perl libxslt dblatex tetex nukeReferences pkgconfig nixUnstable git openssl ];
  [3.60]

  [3.76]
  + [ perl libxslt dblatex tetex nukeReferences pkgconfig nix git openssl ];
• replacement in release.nix at line 74
  [3.10]→[3.10:35](∅→∅)
  − nix = nixUnstable;
  [3.10]

  [3.874]
  + #nix = nixUnstable;