add tmpdir env var export to launch app

[Lunarequest]

This allows for zypak to share the chromium singleton between instances. This is useful when apps are launched via xdg-open from other flatpaks as normally this would start another instance of the app.

[TingPing]

This is no longer needed with flatpak 1.11.1

[Lunarequest]

I've had people report issues unless this was exported

[bbhtt]

It is needed but for other reasons. Electron applications often pack their status icons or whatever inside the asar which tries to extract to /tmp/.chromium-xxxxx but /tmp is unavailable unless explicitly granted access. So they end up going nowhere.

In that case setting TMPDIR to say $XDG_CACHE_HOME/tmp allows it to be self contained while things work.

Giving access to /tmp just for that is a bad idea.

Eg. https://github.com/Heroic-Games-Launcher/HeroicGamesLauncher/blob/f83d036fb83d6302750507714c330cd2c610ef7e/src/backend/constants.ts#L75-L77

[TingPing]

It is complicated because setting TMPDIR to a non-tmpfs ($XDG_CACHE_HOME/tmp) has other implications, like the files will persist forever on disk.

The best solution depends on the specific problem.