Describe the bug

I have a simple script to upload artifacts to cache after a build:

set -x

echo $OUT_PATHS

[[ $DIRENV_IN_ENVRC = 1 ]] && exit
[[ $NO_CACHE_UPLOAD = 1 ]] && exit
[[ -z $OUT_PATHS ]] && exit

export IFS=' '

echo "Signing paths" $OUT_PATHS
nix store sign --key-file secrets/nix-secret-key-file $OUT_PATHS
echo "Uploading paths" $OUT_PATHS
exec nix copy --to 'redacted' $OUT_PATHS

I placed the first echo statement at the top for debugging. The script always fails to upload anything because OUT_PATHS is always empty.

Steps To Reproduce

1. Take the script above
2. set it as a post build hook
3. run nix build -L nixpkgs#hello, add a --rebuild flag if the package has already been built (since hook only runs after a local build)
4. notice how the echo statement echos nothing

Expected behavior

man nix.conf says OUT_PATHS should contain a space separated list of built paths.

nix-env --version output

I tried this on the latest nix unstable as well as stable nix 2.4

Additional context

I am setting the post-build-hook in a flake devShell via the NIX_CONFIG variable. This is done because the cache in question is local to this particular flake and it's environment. Not sure if that may somehow be part of the issue.