ci: pin mamba version for now · scikit-hep/uproot5@481279a

The process '/Users/runner/miniconda3/condabin/mamba' failed with exit code 1

Canceling since a higher priority waiting request for Test build-refs/pull/1401/merge exists

The 'defaults' channel might have been added implicitly. If this is intentional, add 'defaults' to the 'channels' list. Otherwise, consider setting 'conda-remove-defaults' to 'true'.