Environment variables can leak into singularity container

Environment variables set in Python process will get passed into the singularity container started by BmiClientSingularity.
This gives problems when env var value points to a path that is not available inside container.

Problem was detected with GDAL_DATA env var in https://github.com/eWaterCycle/ewatercycle/pull/268#issuecomment-936347883 and was worked around in https://github.com/eWaterCycle/ewatercycle/pull/268#issuecomment-938468873, by setting env var inside Dockerfile.

Possible solution could be to start container with `--cleanenv` arg, see https://singularity.hpcng.org/user-docs/3.8/cli/singularity_run.html?highlight=cleanenv#options