Mamba randomly fails on Windows

The "setup Conda environment" frequently fails on Windows:

> Error: The process 'C:\Miniconda3\condabin\mamba.bat' failed with exit code 1

e.g. https://github.com/dask/distributed/runs/5291766660?check_suite_focus=true

Digging in the log, you can read:
> RuntimeError: The process cannot access the file because it is being used by another process.

The issue has already been fixed upstream in mamba 0.20.0: https://github.com/mamba-org/mamba/issues/1058
Mambaforge-latest (4.11.0) is still shipping 0.19.0; when it does bump up the issue should automatically disappear for us (:crossed_fingers:)