i2pd: enable on all unix platforms, enable parallel build

[prusnak]

Motivation for this change

• enable on all unix platforms
• enable parallel build

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.

[r-rmcgibbo]

Result of nixpkgs-review pr 113020 at 4ba0b20 run on x86_64-linux 1

1 package built:

• i2pd

1 suggestion:

• warning: missing-phase-hooks

  installPhase should probably contain runHook preInstall and runHook postInstall.

  Near pkgs/tools/networking/i2pd/default.nix:33:3:

     |
  33 |   installPhase = ''
     |   ^
  

[r-rmcgibbo]

Result of nixpkgs-review pr 113020 at 4ba0b20 run on aarch64-linux 1

1 package built:

• i2pd

1 suggestion:

• warning: missing-phase-hooks

  installPhase should probably contain runHook preInstall and runHook postInstall.

  Near pkgs/tools/networking/i2pd/default.nix:33:3:

     |
  33 |   installPhase = ''
     |   ^