Use latest micromamba (#288)

No idea why, but this fixed our broken micromamba installations during tests

Author: dnerini

.github/workflows/test_pysteps.yml

@@ -36,7 +36,6 @@ jobs:
       - name: Install mamba and create environment
         uses: mamba-org/provision-with-micromamba@main
         with:
-          micromamba-version: 0.20.0
           environment-file: ci/ci_test_env.yml
           environment-name: test_environment
           extra-specs: python=${{ matrix.python-version }}