Outputs are not propagated to next steps

[sir4ur0n]

Describe the bug
Outputs are not propagated to next steps

To Reproduce

name: Whatever

on:
  workflow_dispatch:

jobs:
  whatever:
    runs-on: [self-hosted, nixos]
    steps:
      - uses: actions/checkout@v4

      - name: a
        id: a
        uses: tweag/run-nix-shell@v0
        with:
          pure: false
          run: |
            echo "foo_bar=what" >> "$GITHUB_OUTPUT"

      - name: b
        id: b
        run: |
          echo "${{ steps.a.outputs.foo_bar }}"

Expected behavior
what should be displayed in the Github Action logs.
If we do the same thing without tweag/run-nix-shell then it works as expected.

Environment

• OS name + version: NixOS 24.05pre-git

[avdv]

That seems to be a limitation of using a composite action: https://github.com/orgs/community/discussions/10529

So, while Javascript actions do not have to declare where an output's value comes from, a composite action must do so and can only refer to an output of one of their steps to return.

AFAICS, the only way to support his is to rewrite the action in Javascript. That should be quite easy, actually.