support a working-directory input

[mvdan]

Just like https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idstepsrun.

Note that trying to use that step field directly on a custom action fails:

     - name: bump go.mod go version if needed
       working-directory: ${{ env.TARGET_REPO_DIR }}
       uses: protocol/multiple-go-modules@v1.1

The workflow is not valid. .github/workflows/copy-workflow.yml (Line: 96, Col: 7): Unexpected value 'uses' .github/workflows/copy-workflow.yml (Line: 97, Col: 7): Unexpected value 'with'