flatpak-dir: Add a debug message to remove_old_appstream_tmpdirs()
flatpak-dir: Add a debug message to remove_old_appstream_tmpdirs()

This should make it a bit clearer when `rm -rf` is being used in the
debug logs.

Signed-off-by: Philip Withnall <12d956b70be7f0d60e54e48e97a01dac38dce989@endlessos.org>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak