install conda as a library in the minimum dependency check CI (#4792)

* install conda as a library

* use python 3.8 instead of 3.9

as it seems conda is not ready for it, yet.

* install py38 when initially setting up the environment

Author: keewis

.github/workflows/ci-additional.yaml

@@ -180,9 +180,10 @@ jobs:
           channel-priority: strict
           mamba-version: "*"
           auto-update-conda: false
+          python-version: "3.8"
 
       - name: minimum versions policy
         run: |
-          mamba install -y pyyaml
+          mamba install -y pyyaml conda
           python ci/min_deps_check.py ci/requirements/py37-bare-minimum.yml
           python ci/min_deps_check.py ci/requirements/py37-min-all-deps.yml