[python/ci] replace miniconda with miniforge (#11085)

With license changes to conda/miniconda, we want to move to miniforge,
which is oss+community run. There's nothing in the github actions
marketplace for miniforge ([one exists but is
archived](https://github.com/conda-forge/setup-miniforge/tree/master)),
so I wrote out their steps for installing the latest version in CI.

Miniforge is the community version of conda, which does not have the
same licensing restrictions as miniconda and automatically uses to
community-run package distributor conda-forge.

### Release Notes

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#### New Features

- N/A

#### Bug Fixes

- #11088 Use miniforge in
Python CI.


### QA Notes

ci should be green

Author: isabelizimm

.github/workflows/positron-python-ci.yml

@@ -357,10 +357,12 @@ jobs:
           }
 
       - name: Install miniconda
-        uses: conda-incubator/setup-miniconda@v3
+        shell: bash
         if: matrix.os == 'macos-latest' && matrix.test-suite == 'venv'
-        with:
-          python-version: "3.11"
+        run: |
+          curl -fsSLo Miniforge3.sh "https://github.com/conda-forge/miniforge/releases/latest/download/Miniforge3-MacOSX-$(uname -m).sh"
+          bash Miniforge3.sh -b -p "${HOME}/conda"
+          source "${HOME}/conda/etc/profile.d/conda.sh"
 
       - name: Prepare conda for venv tests
         env: