DETECTOR_PATH should point to CMAKE_INSTALL_PREFIX subtree, not source dir

Right now DETECTOR_PATH is set (in env.sh) to ${LOCAL_PREFIX}/detector which is where the detector geometry repository is cloned into.

## detector prefix: prefix for the detector definitions
export DETECTOR_PREFIX="${LOCAL_PREFIX}/detector"
mkdir -p ${DETECTOR_PREFIX}

## detector path: actual detector definition path
export DETECTOR_PATH="${DETECTOR_PREFIX}/${JUGGLER_DETECTOR}"

That means any make install doesn't actually get used.