Fix mamba install in conda ci workflow

Author: tobiasdiez

.github/workflows/ci-conda.yml

@@ -61,7 +61,22 @@ jobs:
           key:
             ${{ runner.os }}-conda-${{ hashFiles('src/environment-3.11.yml') }}
 
+      # Mamba currently has problems with dependencies installed from other channels
+      # https://github.com/libarchive/libarchive/issues/1857
+      # https://github.com/conda-incubator/setup-miniconda/issues/292
+      # As temporary workaround we move all preinstalled packages over to conda-forge before installing mamba
       - name: Setup Conda
+        uses: conda-incubator/setup-miniconda@v2
+        with:
+          python-version: ${{ matrix.python }}
+          channels: conda-forge
+          channel-priority: true
+
+      - name: Fix mamba
+        run: |
+          conda upgrade --strict-channel-priority -c conda-forge --all
+
+      - name: Setup Conda environment
         uses: conda-incubator/setup-miniconda@v2
         with:
           python-version: ${{ matrix.python }}