Fix conda build env

Author: philippjfr

.github/workflows/build.yml

@@ -30,6 +30,7 @@ jobs:
       - uses: conda-incubator/setup-miniconda@v2
         with:
           miniconda-version: "latest"
+          python-version: 3.8
       - name: Set output
         id: vars
         run: echo ::set-output name=tag::${GITHUB_REF#refs/*/}