setup-miniconda: Use Mambaforge

The option to use mamba-version in this action is still marked as
experimental and has shown to be problematic with transient failures.

An alternative is to use Mambaforge, which is a Miniforge variant that
comes with Mamba in the base environment. It uses the conda-forge
channel as the only default channel¹.

However, conda-forge must be added to the channels input to the
setup-miniconda action, since it creates and uses a separate condarc
file which contains the specified channels along with defaults², and
defaults should be avoided at least for the Python version³.

¹ https://github.com/conda-forge/miniforge/blob/871e98647a95b99a769dd9f40669d06c44514bca/README.md
² #249 (comment)
³ #249 (comment)

.github/actions/setup-integration-tests/action.yaml

@@ -25,7 +25,7 @@ runs:
     - uses: conda-incubator/setup-miniconda@v2
       with:
         python-version: ${{ inputs.python-version }}
-        mamba-version: "*"
+        miniforge-variant: Mambaforge
         channels: conda-forge,bioconda
 
     - run: cat ~/.profile || true