Enhance artifact upload/download in sync workflow

[rffontenelle]

potodo,md and stats.json were not being updated in the current CI, and that's unintentional. When splitting commit job from sync (translations pull) job, upload-artifact and download-artifact GitHub actions were incorrectly configured:

• upload-artifact action by default warns when no input file is found, and but we want to it to be an error (that shouldn't happen in our scenario).
• upload-artifact action doesn't include hidden files by default, and we need to include the file from hidden directory .tx/config
• upload-artifact action's input files had incorrect path: they are stored inside $PYDOC_LANG_DIR, not in the $GITHUB_WORKSPACE, so the path needed to be specified.
• upload-artifact doesn't keep $PYDOC_LANG_DIR when compressing the input fies, which means in download-artifact needs a 'path' value to be specified in order to extract to the correct directory.