[Common] Only load matLUT once, avoid reload when switching runs (#9171)

Co-authored-by: ALICE Builder <alibuild@users.noreply.github.com>

Author: ddobrigk

Common/TableProducer/trackPropagation.cxx

@@ -146,8 +146,12 @@ struct TrackPropagation {
     }
 
     // load matLUT for this timestamp
-    LOG(info) << "Loading material look-up table for timestamp: " << bc.timestamp();
-    lut = o2::base::MatLayerCylSet::rectifyPtrFromFile(ccdb->getForTimeStamp<o2::base::MatLayerCylSet>(lutPath, bc.timestamp()));
+    if (!lut) {
+      LOG(info) << "Loading material look-up table for timestamp: " << bc.timestamp();
+      lut = o2::base::MatLayerCylSet::rectifyPtrFromFile(ccdb->getForTimeStamp<o2::base::MatLayerCylSet>(lutPath, bc.timestamp()));
+    } else {
+      LOG(info) << "Material look-up table already in place. Not reloading.";
+    }
 
     grpmag = ccdb->getForTimeStamp<o2::parameters::GRPMagField>(grpmagPath, bc.timestamp());
     LOG(info) << "Setting magnetic field to current " << grpmag->getL3Current() << " A for run " << bc.runNumber() << " from its GRPMagField CCDB object";