flatpak-builder: add configuration option for local repository path

[twelho]

Enables the user to configure the local repository path/name for flatpak-builder instead of hard-coding it as repo. This is useful actions workflows that directly export artifacts into a separate, local repository, see for example here. I've renamed localRepoName to localRepoPath in flat-manager as well for consistency and since it should be a real filesystem path, but don't have enough context on how to best solve the FIXME in there.