Referencing own repository in uses tag fails

Trying to use a variable as the name of the repository in a test workflow as follows:

uses: ${{ github.repository }}@HEAD.

This appears to fail with Invalid workflow file

The workflow is not valid.
Unrecognized named-value: ‘github’. Located at position 1 within expression: github.repository

How to reference own repository in a workflow? For a forked project, it makes sense to abstract from the originating repository, especially with modified action code. Otherwise the tests will run against the action code in the parent repository, not the forked one.