release.nix: Remove `services' arguments to ${nixos}/lib/testing.nix.

Dependencies

• [2] 7C6HSXUQ * Start of a regression test for Hydra.
• [3] YQWH4POV * Simplify.
• [4] Y2BFYLLK * Install the SQL schemata in $out/share/hydra/sql.
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 85
  [2.162]→[2.162:251](∅→∅)
  − with import "${nixos}/lib/testing.nix" { inherit nixpkgs system; services = null; };
  [2.162]

  [2.251]
  + with import "${nixos}/lib/testing.nix" { inherit nixpkgs system; };