Multiple tmp dirs created but only one used for file-backed dataset store

This only applies to the case when the data processing has the dataset store backed by an hdf5 file.

In such a case, each process creates a temp dir to hold an hdf5 file: https://github.com/DiamondLightSource/httomo/blob/2acd1b2278a337458a00b63e8528117892eba647/httomo/cli.py#L428-L438

but when the writer actually defines the hdf5 filepath, only rank 0's temp dir is used: https://github.com/DiamondLightSource/httomo/blob/2acd1b2278a337458a00b63e8528117892eba647/httomo/data/dataset_store.py#L174

Meaning, each process creates a temp dir when really only rank 0 needs to create one.

This doesn't have any impact on the functionality, it's simply just a bit confusing when seeing multiple temp dirs created and only one is actually being used.