How do I call a GitHub reusable workflow from the same branch?

I have a repository that contains some custom actions and reusable workflows. In order to test any changes to those artefacts I would like to call them from a workflow in the same repository and verify that they behave as expected.

One important point here is that I want to call them from the same branch that the PR is running for. The reason is simple, I want to test the changes that I am making in the current branch.

Here is what I have come up with:

---
    name: 'Test custom actions and reusable workflows'

    on:
      pull_request:
        branches:
          - main
  
    jobs:
      call-custom-action:
        runs-on: [ self-hosted, default ]
        steps:
          - name: get env vars
            id: get-env-vars
            # owner/repo is the repo that this workflow resides in
            uses: owner/repo/actions/my-custom-action@${{ env.GITHUB_HEAD_REF }}
  
      call-reusable-workflow:
        uses: owner/repo/.github/workflows/my-reusable-workflow.yml@${{ env.GITHUB_HEAD_REF }}

When I commit and push this workflow it fails with error:

> Invalid workflow file: .github/workflows/test-reusable-workflow.yml#L19.
> error parsing called workflow
".github/workflows/test-reusable-workflow.yml"
> -> "owner/repo/.github/workflows/my-reusable-workflow.yml@${{env.GITHUB_HEAD_REF}}"
> : failed to fetch workflow: reference to workflow should be either a valid branch, tag, or commit

I tried changing the offending line to

        uses: owner/repo/.github/workflows/my-reusable-workflow.yml

but that failed with:

> invalid value workflow reference: no version specified

As can be observed there doesn't seem to be a way to call a reusable workflow by referring to the version using an expression. Is there a way to call reusable workflow from the same branch?

You can call a workflow on the currently checked out branch by providing a relative path (see docs):

        uses: ./.github/workflows/my-reusable-workflow.yml