Install Miniforge explicitly in CI

Author: tobiasraabe

.github/workflows/main.yml

@@ -28,10 +28,13 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: r-lib/actions/setup-tinytex@v2
-      - uses: conda-incubator/setup-miniconda@v2
+      - uses: conda-incubator/setup-miniconda@v3
         with:
+          miniforge-version: latest
           auto-update-conda: true
           python-version: ${{ matrix.python-version }}
+          channels: conda-forge
+          conda-remove-defaults: true
 
       - name: Install core dependencies.
         shell: bash -l {0}