Like $XDG_RUNTIME_DIR/app/$FLATPAK_ID, this is shared between all
instances of the app, except for subsandboxed instances created by
flatpak-spawn --sandbox or equivalent. Unlike
$XDG_RUNTIME_DIR/app/$FLATPAK_ID, it does not exist at an equivalent
path on the host and in the sandboxed app.
Resolves: https://github.com/flatpak/flatpak/issues/4120
Signed-off-by: Simon McVittie <smcv@collabora.com>