utils: Remap /run/flatpak/app, for Flatpak 1.11.x

Flatpak 1.11.x avoids mounting anything into the app-controlled
XDG_RUNTIME_DIR, so $XDG_RUNTIME_DIR/app in the container shows up as
a symlink to /run/flatpak/app. This particularly affects apps that
set TMPDIR=$XDG_RUNTIME_DIR/app/$FLATPAK_ID and use the URI portal
to request that a file in /tmp gets opened on the host.

Manual test:

    (run any Flatpak app with --command=bash)
    echo "hello" > $XDG_RUNTIME_DIR/app/$FLATPAK_ID/readme.txt
    xdg-open $XDG_RUNTIME_DIR/app/$FLATPAK_ID/readme.txt

Resolves: #589
Signed-off-by: Simon McVittie <smcv@collabora.com>