Set miniforge-variant for conda CI

Author: domdfcoding

.github/workflows/conda_ci.yml

@@ -32,6 +32,7 @@ jobs:
           activate-environment: env
           conda-build-version: 3.21.0
           python-version: "3.8"
+          miniforge-variant: Mambaforge
 
       - name: Install dependencies 🔧
         run: |