Motivation for this change

⚠️ I only tested the migration in a local setup and not yet on my Hydra instance.

Updates Hydra to the latest master (and flake) revision to incorporate the massive performance improvements.

Before this is mergable, the following things should be discussed:

• I had to split the sources for several hydra variants (a.k.a. flake-support, unstable and stable), however hydra-stable is currently broken since Hydra doesn't compile with nix 2.3. For now, I marked it as broken (since upstream uses pkgs.nixFlakes on Hydra), however we could fix it:
  • Pin Hydra with nix-stable to an older revision (and risking that it lacks appropriate maintenance).
  • Patch Hydra to support Nix 2.3 again (I actually attempted to do this today but this requires a bit more effort).
• A few weeks ago I was asked if this should be backported to 20.03. I think that it should since Graham provided pretty useful migration instructions and I played around with it locally and the migration feels pretty smooth. However that's up to our release managers.
• (out of scope) We should consider marking this package as deprecated for 20.09 or 21.03 (depending on when flakes will be stabilized) as there's no point in maintaining this package (in addition to the Nix expressions in nixos/hydra) as soon as flakes are out.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.