gh-36468: Fix mamba install in conda ci workflow
    
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Mamba currently has problems with dependencies installed from other
channels (libarchive/libarchive#1857,
conda-incubator/setup-miniconda#292,
conda-forge/libarchive-feedstock#69), see eg h
ttps://github.com/sagemath/sage/actions/runs/6525067702/job/17717363515.

As workaround we use miniforge now (which comes with a preinstalled
mamba from conda-forge)

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URL: #36468
Reported by: Tobias Diez
Reviewer(s): Isuru Fernando, Matthias Köppe

Author: Release Manager

.github/workflows/ci-conda.yml

@@ -39,18 +39,6 @@ jobs:
         env:
           GH_TOKEN: ${{ github.token }}
 
-      - name: Check for Miniconda
-        id: check_conda
-        run: echo ::set-output name=installed::$CONDA
-
-      # Miniconda is installed by default in the ubuntu-latest, however not in the act-image to run it locally
-      - name: Install Miniconda
-        if: steps.check_conda.outputs.installed == ''
-        run: |
-          wget https://repo.anaconda.com/miniconda/Miniconda3-py39_4.10.3-Linux-x86_64.sh -O ~/miniconda.sh
-          bash ~/miniconda.sh -b -p $HOME/miniconda
-          echo "CONDA=$HOME/miniconda" >> $GITHUB_ENV
-
       - name: Create conda environment files
         run: ./bootstrap-conda
 
@@ -61,12 +49,13 @@ jobs:
           key:
             ${{ runner.os }}-conda-${{ hashFiles('src/environment-3.11.yml') }}
 
-      - name: Setup Conda
+      - name: Setup Conda environment
         uses: conda-incubator/setup-miniconda@v2
         with:
           python-version: ${{ matrix.python }}
-          mamba-version: "*"
-          channels: conda-forge,defaults
+          miniforge-version: latest
+          use-mamba: true
+          channels: conda-forge
           channel-priority: true
           activate-environment: sage
           environment-file: src/${{ matrix.conda-env }}-${{ matrix.python }}.yml