epoch -> edition

Dependencies

• [2] 6LB22PZN Add a NixOS container to demo Hydra
• [3] P3M6UFMP Merge remote-tracking branch 'origin/master' into flake
• [4] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in flake.nix at line 4
  [3.58]→[3.58:76](∅→∅)
  − epoch = 201909;
  [3.58]

  [2.217]
  + edition = 201909;