UN demand data inputs removed automatically for each run of build_base_energy_totals

[ekatef]

Checklist

• I am using the current main branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-earth environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

It appears that an explicit specification of the the UN data directory as snakemake output needed to improve stability the workflow #1279 had also some side effects.

If build_base_energy_totals is being executed Snakemake first removes all the outputs including data/demand/unsd/data/. If demand_data:update_data is false the execution is being terminated, otherwise the UN demand files are being downloaded again.
It feels like the behaviour is linked with Snakemake logic, and a way to fix may be to breakdown build_base_energy_totals on the data retrieval and the computational parts.