PyPI publish GitHub Action vulnerable to injectable expression expansions in action steps

Summary

### Summary

`gh-action-pypi-publish` makes use of GitHub Actions expression expansions (i.e. `${{ ... }}`) in contexts that are potentially attacker controllable. Depending on the trigger used to invoke `gh-action-pypi-publish`, this may allow an attacker to execute arbitrary code within the context of a workflow step that invokes `gh-action-pypi-publish`.

### Details

`gh-action-pypi-publish` contains a composite action step, `set-repo-and-ref`, that makes use of expression expansions:

```yaml - name: Set repo and ref from which to run Docker container action id: set-repo-and-ref run: | # Set repo and ref from which to run Docker container action # to handle cases in which `github.action_` context is not set # https://github.com/actions/runner/issues/2473 REF=${{ env.ACTION_REF || env.PR_REF || github.ref_name }} REPO=${{ env.ACTION_REPO || env.PR_REPO || github.repository }} REPO_ID=${{ env.PR_REPO_ID || github.repository_id }} echo "ref=$REF" >>"$GITHUB_OUTPUT" echo "repo=$REPO" >>"$GITHUB_OUTPUT" echo "repo-id=$REPO_ID" >>"$GITHUB_OUTPUT" shell: bash env: ACTION_REF: ${{ github.action_ref }} ACTION_REPO: ${{ github.action_repository }} PR_REF: ${{ github.event.pull_request.head.ref }} PR_REPO: ${{ github.event.pull_request.head.repo.full_name }} PR_REPO_ID: ${{ github.event.pull_request.base.repo.id }} ```

Permalink: https://github.com/pypa/gh-action-pypi-publish/blob/db8f07d3871a0a180efa06b95d467625c19d5d5f/action.yml#L114-L125

In normal intended operation, these expansions are used to establish a correct priority for outputs like `ref` and `repo-id`.

However, these expansions have a side effect: because they're done with `${{ ... }}` and not with `${...}` (i.e. normal shell interpolation), they can *bypass normal shell quoting rules*. In particular, if both `env.ACTION_REF` and `env.PR_REF` evaluate to empty strings, then the expression falls back to `github.ref_name`, which can be an attacker controlled string via a branch or tag name.

For example, if the attacker is able to set a branch name to something like `innocent;cat${IFS}/etc/passwd`, then the `REF` line may expand as:

```bash REF=innocent;cat${IFS}/etc/passwd ```

which would set `REF` to `innocent` and then run the attacker's code.

Additional information about dangerous expansions can be found in [zizmor's `template-injection` rule documentation](https://woodruffw.github.io/zizmor/audits/#template-injection).

### Impact

The impact of this vulnerability is very low: the expression in question is unlikely to be evaluated in normal operation, since `env.ACTION_REF` should always take precedence.

In particular, the action is **not** vulnerable in many popular configurations, i.e. those where `pull_request` or `release` or a `push: tags` event is used to call the action.