CI: install miniforge not mambaforge

merges manubot#520
refs manubot#519

miniforge installer now includes mamba.

Author: habi

.github/workflows/manubot.yaml

@@ -84,9 +84,6 @@ jobs:
           activate-environment: manubot
           environment-file: build/environment.yml
           auto-activate-base: false
-          miniforge-variant: Mambaforge
-          miniforge-version: 'latest'
-          use-mamba: true
       - name: Install Spellcheck
         if: env.SPELLCHECK == 'true'
         run: bash ci/install-spellcheck.sh