chore(ci): switch to miniforge (#1304)

* Switched to miniforge

* ci: restore activation

* ci: verify Python version

* Update build-test.yml

* ci: use mamba (15 seconds faster)

* Update build-test.yml

---------

Co-authored-by: Jim Pivarski <jpivarski@users.noreply.github.com>
Co-authored-by: Henry Schreiner <henry.fredrick.schreiner@cern.ch>

.github/workflows/build-test.yml

@@ -34,9 +34,12 @@ jobs:
         with:
           auto-update-conda: true
           python-version: ${{ matrix.python-version }}
-          miniforge-variant: Mambaforge
+          miniforge-version: latest
           use-mamba: true
 
+      - name: Check active Python version
+        run: python -c "import sys; assert '.'.join(str(s) for s in sys.version_info[:2]) == '${{ matrix.python-version }}', f'{version} incorrect!'"
+
       - name: Install ROOT
         if: matrix.python-version == 3.8  &&  runner.os != 'macOS'  &&  runner.os != 'Windows'
         run: |