ci: try better self-hosted mamba env

.github/workflows/test_suite.yaml

@@ -77,17 +77,9 @@ jobs:
         with:
           fetch-depth: 0
 
-      - uses: conda-incubator/setup-miniconda@v3
-        with:
-          python-version: ${{ env.PYTHON }}
-          mamba-version: "*"
-          channels: conda-forge,defaults
-          channel-priority: true
-          activate-environment: testenv
-          environment-file: ci/test-env.yml
-
       - name: Install
         run: |
+          micromamba activate testenv
           echo $(which pip)
           python -m pip install --upgrade pip
           python -m pip install .[test,gpu]