exportReferencesGraph doesn't work in `nix-shell`

[arianvp]

Describe the bug

I'd expect the references graph to be in TMPDIR but it's not

Steps To Reproduce

{
  cpio,
  runCommand,
  systemd,
}:
runCommand "initrd"
  {
    name = "initrd";

    exportReferencesGraph = [
      "systemd-graph"
      systemd
    ];

    # unsafeDiscardReferences.out = true;
  }
  ''
    ls .
  ''

Expected behavior

ls $TMPDIR

shows a systemd-graph file but it doesn't

Metadata

Additional context

Checklist

• [ ] checked latest Nix manual (source)
• [ ] checked open bug issues and pull requests for possible duplicates

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[Ericson2314]

This didn't work in the past to your knowledge did it? (Just trying to understand is this, strictly speaking, a feature request or a regression.)

[roberth]

Does this apply to both structuredAttrs and without?

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2025-06-11-nix-team-meeting-minutes-231/65543/1