Use nixUnstable for the tarball

Dependencies

• [2] FJAF7KWA Disable building the PDF manual
• [3] NRNR4QVJ Use stable Nix
• [4] R6O64PAE Use a nicer version string
• [5] LOHWNXEJ Cleanup
• [6] 4C5F6SZV add openssl to deps
• [7] M3A5PZIH hydra: Clarify the dependency on BDW-GC.
• [8] 6NWLMLSS remove nix from buildinputs
• [9] V2ALOFRR Fix the test
• [10] UHMUHQMU hydra: fix tarball build, add pre suffix to tarballs
• [11] WYM574OI add nix to buildinputs
• [12] KNLKTCDM Use pkgconfig to find Nix
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 41
  [3.60]→[2.29:96](∅→∅)
  − [ perl libxslt nukeReferences pkgconfig nix git openssl ];
  [3.60]

  [3.76]
  + [ perl libxslt nukeReferences pkgconfig nixUnstable git openssl ];