RootAuxDynIO: Disable buffered RNTuple writing

Running some multi-threaded reconstruction test [1] where we write out AODs w/ RNTuples I noticed that the job crashes (after processing a hundred or so events) fairly consistently.

Looking at the stack trace, I suspected this might be due to the default buffered writing. Turning it off indeed seems to help - without any apparent major downside, at least for me locally.

[1] Here is the reproducer:

lsetup "asetup Athena,main--dev3LCG,latest";
ATHENA_CORE_NUMBER=8 Reco_tf.py \ 
  --preExec 'flags.Output.StorageTechnology="ROOTRNTUPLE";' \ 
  --CA 'True' \ 
  --perfmon 'fullmonmt' \ 
  --inputBSFile '/eos/atlas/atlascerngroupdisk/proj-spot/spot-job-inputs/data23_13p6TeV/data23_13p6TeV.00451569.physics_Main.daq.RAW._lb0260._SFO-14._0001.data' \ 
  --maxEvents '1000' \ 
  --outputAODFile 'myAOD.pool.root' \ 
  --outputHISTFile 'myHIST.root' \ 
  --multithreaded 'True' \ 
  --autoConfiguration 'everything' \ 
  --conditionsTag 'CONDBR2-BLKPA-2023-02' \ 
  --geometryVersion 'ATLAS-R3S-2021-03-02-00' \ 
  --runNumber '451569' \ 
  --steering 'doRAWtoALL';