apparmor-tumbler: allow running from the source tree

We need the compiled LD_PRELOAD hack from the installed tree, but
the rest can come from the source, allowing for quicker test/fix
cycles.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3477