Just added the pre-build-command example given by tarleb and worked! It seems that the newer version of sphinx-action fixed the implementation.

- name: Build HTML  
  uses: ammaraskar/[email protected] #sphinx-action@master  
  # installing pandoc by pre-build-command  
  with:  
    pre-build-command: \>-  
      apt-get update && apt-get install -y pandoc

Thanks a lot!