Install mamba in appropriate test job

Author: fcollonval

.github/workflows/build.yml

@@ -84,6 +84,7 @@ jobs:
         run: |
           conda activate test_gator
           conda install -n test_gator --file requirements_dev.txt
+          conda install -n test_gator mamba
           python setup.py develop --skip-npm
           # Check pip dependencies
           python -m pip check
@@ -211,7 +212,6 @@ jobs:
           activate-environment: test_gator
           auto-update-conda: true
           channels: conda-forge
-          mamba-version: "*"
           python-version: "3.7"
           show-channel-urls: true
           use-only-tar-bz2: true
@@ -220,7 +220,7 @@ jobs:
         run: |
           conda activate test_gator
           mamba install -n test_gator --file requirements_dev.txt
-          conda install -n test_gator coveralls jupyterlab=3
+          conda install -n test_gator coveralls jupyterlab=3 mamba
           yarn install
           python -m pip install -e .
           # Check pip dependencies