launch-client: Default to --directory="", except for Flatpak

Based on !475 (merged), which makes the documentation for --directory describe its current behaviour correctly.

• launch-client: Default to --directory="", except for Flatpak

  When we are behaving like flatpak-spawn(1), its behaviour of inheriting the current working directory makes sense: flatpak-spawn is meant to run a command with different sandboxing, but as though it was a child of the current process. Also, the current working directory for the flatpak-portal and flatpak-session-helper services is uninteresting (99% of the time it will be the user's home directory), so it will not usually be useful to inherit.

  However, when we are inserting commands into a running game, it makes more sense to inherit the game's current working directory, which is non-trivial (usually but not always the game's top-level installation directory). Change s-r-launch-client so it does that by default, instead of forcing its own working directory.

  steamrt/tasks#145