Runner.workspace not available in env:

I’m trying to set an environment variable in a simple-crossplatform way:

    env:
      CACHE_DIR: ${{runner.workspace}}/mudlet-build-cache

But it would seem that does not work due Unrecognized named-value: 'runner'. Located at position 1 within expression: runner.workspace .

${{runner.workspace}} works in other contexts fine, so why not here as well?