Allow running on i686-linux

Dependencies

• [2] WQ2VQ7H3 Use hydra-module.nix in the tests
• [3] LOHWNXEJ Cleanup
• [4] IZYR3KTL release.nix: Get rid of the "system" argument
• [5] XWNMTW6P Drop building on i686-linux
• [6] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 9
  [3.37]→[3.0:52](∅→∅)
  − genAttrs' = pkgs.lib.genAttrs [ "x86_64-linux" ];
  [3.37]

  [2.0]
  + genAttrs' = pkgs.lib.genAttrs [ "x86_64-linux" "i686-linux" ];