fix(snakefile): correctly locate files to translate

[AmitMY]

Similar to #402

In multiple places (e.g. https://github.com/mozilla/firefox-translations-training/blob/main/Snakefile#L549) there is an explicit file path to a .gz extension. While I use .gz, the split command supports other extensions as well. I think this file path should be generic (file.{{part}}.{{ext}}), but don't know how to make it as such.