Fix macOS CI: set CONDA_OVERRIDE_OSX to resolve libmamba codesign failure (#1239)

* Initial plan

* Fix macOS CI: set CONDA_OVERRIDE_OSX=12.0 to prevent codesign failure

* Fix macOS CI: apply CONDA_OVERRIDE_OSX only on macOS via conditional step

* Fix macOS CI: use miniforge instead of mamba to avoid codesign Broken Pipe failure

* Restore strict channel priority for reproducible conda builds

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

.github/workflows/python-package-conda.yml

@@ -21,12 +21,12 @@ jobs:
       - name: Create python 3.11 conda env
         uses: conda-incubator/setup-miniconda@v3
         with:
-          python-version: 3.11
-          mamba-version: "*"
+          python-version: '3.11'
+          miniforge-version: latest
           activate-environment: donkey
           auto-activate-base: false
-          channels: default, conda-forge, pytorch
-          channel-priority: true
+          channels: conda-forge, pytorch
+          channel-priority: strict
       - name: Conda info and list
         run: |
           conda info