Run the l10n-sync script in check-only mode for every PR

Currently we have l10n-sync script that runs every day to sync l10n and main branches. Sometimes when we do a change in that directory it could break this sync. So instead we should run this script in the dry-run mode for every PRs to make sure that we are not breaking it. It would also be useful for new contributors since they tend to change non en-US locales as well.
This came up from the discussion in #4344.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-594)