Detector update fix

Fixing the updating problem. https://eicweb.phy.anl.gov/EIC/benchmarks/reconstruction_benchmarks/-/jobs/59417#L52

Is this caused by the job artifact or caching? @sly2j

util/build_detector.sh

@@ -33,7 +33,8 @@ if [ ! -d ${JUGGLER_DETECTOR} ]; then
 else
   echo "Updating ${JUGGLER_DETECTOR}"
   pushd ${JUGGLER_DETECTOR}
-  git pull --ff-only
+  git fetch origin
+  git checkout -b ${JUGGLER_DETECTOR_VERSION}
   popd
 fi
 ## We also need an up-to-date copy of the accelerator. For now this is done