CI: Install libmamba from conda-forge

.github/workflows/ci_np2.yml

@@ -20,6 +20,7 @@ jobs:
         echo $CONDA/bin >> $GITHUB_PATH
     - name: Install dependencies
       run: |
+        conda install -c conda-forge libmamba
         conda env update --file environment_np2.yml --name base
     - name: Conda info
       shell: bash -l {0}