Describe the bug

As part of https://github.com/JulienMalka/lila, we have a post-build-hook that posts an attestation of the build to the hash collection service. Since a recent nixpkgs update, this doesn't work for me anymore. I haven't properly bisected yet, but it seems it still worked on nix 2.29.2 and fails on nix 2.30.3

Steps To Reproduce

• register a post-build-hook that prints the OUT_PATHS environment variable
• nix-build '<nixpkgs>' -A nethogs --no-out-link; nix-build '<nixpkgs>' -A nethogs --check --no-out-link or nix build nixpkgs#nethogs; nix build nixpkgs#nethogs --rebuild.

The post-build-hook is called for the rebuild, but OUT_PATHS appears to be unset

Expected behavior

The post-build-hook should be called for the rebuild with OUT_PATHS pointing to the out paths

Metadata

nix-env (Nix) 2.31.2

Also verified it exists on Nix 2.32pre20250919_07b96c1d

Additional context

This is on nixos. Issue in lila is JulienMalka/lila#66 .

#13802 looks potentially related but isn't in 2.31.2 yet so that's unlikely to be it.

Maybe #13769?

Checklist

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

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