fix fastlane metadata in a subdir losing its images and changelogs

insert_localized_app_metadata checks a build's subdir based on the path
suffix, but this only works for the base locale dir (with the summary,
descriptiion, etc), not the images/changelogs subdir, which are silently
ignored.

Triple-T metadata does not have this issue since copy_triple_t_store_metadata
finds the base metadata dir, then goes from there.

Searching for the fastlane dir from the end of the path instead of using
a fixed offset fixes this.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f