Allow nested secrets in `secrets.json`

[srid]

Prior to this change, if secrets.json had nested secrets (example) we would see this error:

sops-install-secrets: Manifest is not valid: secret jenkins-nix-ci/cachix-auth-token/description in /nix/store/wxm763za3rbrpiijfbgss9g5ll0sd29z-secrets.json is not valid: Key 'jenkins-nix-ci' does not refer to a dictionary

The reason happens to be that introspecting the map key to be interface fails, when it is in fact a string. This PR makes it so that we always expect the key to be a string (what else could it be?). It also improves the error message, by telling the user what the actual value type is.

[srid]

Huh, this is causing the yaml processing to regress.

[shivaraj-bh]

I am trying to write the test for this. Here's how the nixos-test.nix looks:

nested-json = makeTest {
    name = "sops-nested-json-secrets";
    nodes.server = {
      imports = [ ../../modules/sops ];
      sops = {
        age.keyFile = ./test-assets/age-keys.txt;
        defaultSopsFile = ./test-assets/secrets.json;
        secrets."nested/test/file" = { };
      };
    };

    testScript = ''
      start_all()
      server.succeed("cat /run/secrets/nested/test/file | grep -q 'another value'")
    '';
  } {
    inherit pkgs;
    inherit (pkgs) system;
  };

Expected: testScript should fail with Key 'nested' does not refer to a dictionary Actual: Test passes On the other hand, if I try to run nix build .#nixosConfigurations.actual.config.system.build.toplevel on @srid 's nixos-config with sops-nix.url pointing to github:Mic92/sops-nix, I am able to reproduce the same error. @Mic92 Is there something that's happening under-the-hood in nixos-test.nix that I might be missing?