Update DIaL3 rose stem config

[dennissergeev]

The export_source task has been changed since vn1.1 - it is now a python script which uses subprocess.run(), with capture_output=True as an argument. This requires python>=3.7.

We use Python 3.8 in the software stack (/scratch/dp015/dc-uoeum/UoEGCC1120SWstackBuild/RoseStemSource.sh) but this needs to be sourced in the [[DIAL3_EXPORT-SOURCE]] task in rose-stem/site/uoe/common/suite_config_dial3.cylc. Or at least we should add module load python/3.8.10 to that task's environment.

For now the workaround is to add module load python/3.8.10 to ~/.bashrc.