Make the action compatible with sphinx-multiversion

[snorkman88]

I have tried replacing the custom build command with:

      - name: Build docs using action
        uses: ammaraskar/sphinx-action@master
        with:
          docs-folder: "docs/source/"
          build-command: "pip install sphinx-multiversion && sphinx-multiversion . docs/_build"

but it seems it is not possible to use this action with sphinx-multiversion yet, would it be possible to support such feature?