Builds for attributes which are not derivations but instantiate ok "succeed" and produce metadata but no log

See https://github.com/NixOS/nixpkgs/pull/37218

[nix-shell:~/projects/nixpkgs]$ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate ./nixos/release.nix -A tests.kubernetes.rbac --option restrict-eval true --option build-timeout 1800 --arg supportedSystem '["x86_64-linux"]' --show-trace

[nix-shell:~/projects/nixpkgs]$ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate ./nixos/release.nix -A tests.kubernetes.dns --option restrict-eval true --option build-timeout 1800 --arg supportedSystem '["x86_64-linux"]' --show-trace

so evaluation "Succeeds" so they "build" which also "succeeds" but then there is nothing to do. This also meanss the builder sendss a "started building" job and a "result" job but no log file is created because no lines were sent.

See:

    "ea8221cb-2bc3-4d8c-822c-3c421f7f2a41": {
      "result": {
        "repo": {
          "owner": "NixOS",
          "name": "nixpkgs",
          "full_name": "NixOS/nixpkgs",
          "clone_url": "https://github.com/NixOS/nixpkgs.git"
        },
        "pr": {
          "target_branch": "master",
          "number": 37218,
          "head_sha": "e359258c739b42415ecc46416c466bb515effc53"
        },
        "system": "x86_64-linux",
        "output": [],
        "attempt_id": "ea8221cb-2bc3-4d8c-822c-3c421f7f2a41",
        "success": true,
        "skipped_attrs": [],
        "attempted_attrs": [
          "tests.kubernetes.dns",
          "tests.kubernetes.rbac"
        ]
      },
      "metadata": {
        "system": "x86_64-linux",
        "identity": "gleber-bastion",
        "attempt_id": "ea8221cb-2bc3-4d8c-822c-3c421f7f2a41",
        "attempted_attrs": [
          "tests.kubernetes.dns",
          "tests.kubernetes.rbac"
        ],
        "skipped_attrs": []
      }
    }

Start by tracing ofborg's builder handling for nix-instantiate results, using the commands and JSON attempt metadata in the issue as the reproduction case. Inspect how started-building and result jobs are handled when output is empty, then verify that these attempts receive an appropriate log or status instead of appearing successful with no work.