feat: skip tranlations for repository plugins

Maybe it would be better to split workflow for paid and free plugins
(and definitely for libraries), but for now it should be enough to skip
prepare translations job and mark it optional in other stages.

Translations for free plugins are handled by WordPress repository, thus
there's no need to keep this job. Essentially, most plugins targeting
WordPress repository doesn't have its own translation files either.

Signed-off-by: Bart Jaskulski <bjaskulski@protonmail.com>