Include _ERT_EXPERIMENT_ID in case metadata

[perolavsvendsen]

_ERT_EXPERIMENT_ID is exposed as an environment variable ref this table from #605

ERT mode	On client	On Compute node
workflow	_ERT_EXPERIMENT_ID _ERT_ENSEMBLE_ID _ERT_SIMULATION_MODE	N/A
test-run (forward model)	_ERT_EXPERIMENT_ID _ERT_ENSEMBLE_ID _ERT_SIMULATION_MODE _ERT_RUNPATH _ERT_ITERATION_NUMBER _ERT_REALIZATION_NUMBER	N/A
ensemble experiment (forward model)	N/A	_ERT_RUNPATH _ERT_ITERATION_NUMBER _ERT_REALIZATION_NUMBER

We should include this in the outgoing case metadata, to learn more about the likeliness between experiment and case

In the metadata, suggest we put this into a separate block:

data:
  ...
fmu:
  case:
    name: ...
    uuid: ...
  ert:
    experiment:
      id: <_ERT_EXPERIMENT_ID>  # if it is a uuid, use "uuid"