refs #13997. Don't rewrite spectra-detector map

In the algorithm. David has explicitly set the spectrum numbers, so don't overwrite what he's put there already.

Framework/DataHandling/src/LoadBBY.cpp

@@ -405,6 +405,7 @@ void LoadBBY::exec() {
       createChildAlgorithm("LoadInstrument");
   loadInstrumentAlg->setProperty("Workspace", eventWS);
   loadInstrumentAlg->setPropertyValue("InstrumentName", "BILBY");
+  loadInstrumentAlg->setProperty("RewriteSpectraMap", false);
   loadInstrumentAlg->executeAsChildAlg();
 
   setProperty("OutputWorkspace", eventWS);
@@ -826,4 +827,4 @@ void BbyDetectorBankFactory::createAndAssign(size_t startIndex,
 }
 
 } // DataHandling
-} // Mantid
\ No newline at end of file
+} // Mantid