darwin: include system paths with age plugin PATH

[tiferrei]

Fixes #957.

---

Minimal reproducing example:

{
  description = "Minimal sops-nix issue 957 repro";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    nix-darwin.url = "github:nix-darwin/nix-darwin";
    nix-darwin.inputs.nixpkgs.follows = "nixpkgs";
    sops-nix.url = "path:/Users/tiferrei/Developer/sops-nix";
  };

  outputs =
    {
      nix-darwin,
      sops-nix,
      ...
    }:
    {
      darwinConfigurations.test = nix-darwin.lib.darwinSystem {
        modules = [
          sops-nix.darwinModules.sops
          (
            { pkgs, ... }:
            {
              nixpkgs.hostPlatform = "aarch64-darwin";
              documentation.enable = false;
              system.stateVersion = 5;

              sops.validateSopsFiles = false;
              sops.defaultSopsFile = "/dev/null";
              sops.age.keyFile = "/tmp/sops-nix-test-age-key.txt";
              sops.age.plugins = [ (pkgs.writeShellScriptBin "age-plugin-test" ":") ];
              sops.secrets."issue-957" = { };
            }
          )
        ];
      };
    };
}

Currently, the produced launchd daemon command lacks needed system paths:

nix eval --raw .#darwinConfigurations.test.config.launchd.daemons.sops-install-secrets.command

sh -c '
echo "Setting up secrets..."
(
# shellcheck disable=SC2030,SC2031
  export PATH='\''/nix/store/hy2qqz7kfz0y5kind56lxa6p8rby6jck-age-plugin-test/bin'\''
  export SOPS_GPG_EXEC='\''/nix/store/cgh6iwzz5jgx9z5whka4vgj210i6npc6-gnupg-2.4.9/bin/gpg'\''
  /nix/store/mm1ncsav8d2is67r6jqkaw5b87kbr638-sops-install-secrets-0.0.1/bin/sops-install-secrets /nix/store/7m5xpp5g7kphqrqr1681530rxd2hrn99-manifest.json
)

'

After the patch (a la #891):

sh -c '
echo "Setting up secrets..."
(
# shellcheck disable=SC2030,SC2031
  export PATH='\''/nix/store/hy2qqz7kfz0y5kind56lxa6p8rby6jck-age-plugin-test/bin:/usr/bin:/bin:/usr/sbin:/sbin'\''
  export SOPS_GPG_EXEC='\''/nix/store/cgh6iwzz5jgx9z5whka4vgj210i6npc6-gnupg-2.4.9/bin/gpg'\''
  /nix/store/mm1ncsav8d2is67r6jqkaw5b87kbr638-sops-install-secrets-0.0.1/bin/sops-install-secrets /nix/store/7m5xpp5g7kphqrqr1681530rxd2hrn99-manifest.json
)

'

[Mic92]

is this not always true?

[tiferrei]

Ah yes, you're right. I mirrored the behaviour of the related fix, but as this is already in nix-darwin scope, it can be simplified. ill fix.