dev: Stop using Mambaforge in CI; the Miniforge3 installer is now equ…

…ivalent

Scheduled CI runs today failed¹ due to a Mambaforge brownout ahead of its
removal in January 2025; it was deprecated in late July 2024.²

¹ <https://github.com/nextstrain/cli/actions/runs/11130647037/job/30930449798#step:3:135>
² <https://conda-forge.org/news/2024/07/29/sunsetting-mambaforge/>

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

@@ -25,7 +25,7 @@ runs:
     - uses: conda-incubator/setup-miniconda@v3
       with:
         python-version: ${{ inputs.python-version }}
-        miniforge-variant: Mambaforge
+        miniforge-version: Miniforge3
         channels: conda-forge,bioconda
 
     - run: cat ~/.profile || true