[ci] Use conda python for m1 jobs (#4351)

There seems to be a discrepancy between testing.yml and release.yml.
For example, my opengl PR #4345 fails at vulkan backend on m1. Also today's
nightly failed. So unifying them here to debug...

.github/workflows/testing.yml

@@ -445,19 +445,21 @@ jobs:
           if [[ ${{needs.check_files.outputs.run_job}} == false ]]; then
             exit 0
           fi
-          rm -rf $HOME/Library/Python/3.8/lib/python/site-packages/taichi
+          export PATH=/Users/github/miniforge3/envs/$PYTHON/bin:$PATH
           brew install molten-vk
           .github/workflows/scripts/unix_build.sh
         env:
           TAICHI_CMAKE_ARGS: -DTI_WITH_OPENGL:BOOL=OFF -DTI_WITH_CUDA:BOOL=OFF -DTI_WITH_CC:BOOL=OFF -DTI_WITH_VULKAN:BOOL=ON -DTI_BUILD_TESTS:BOOL=ON -DCMAKE_C_COMPILER_LAUNCHER=sccache -DCMAKE_CXX_COMPILER_LAUNCHER=sccache
+          PYTHON: ${{ matrix.python }}
           CXX: clang++
 
       - name: Test
         run: |
           if [[ ${{needs.check_files.outputs.run_job}} == false ]]; then
             exit 0
           fi
-          export PATH=$PATH:$HOME/Library/Python/3.8/bin
+          export PATH=/Users/github/miniforge3/envs/$PYTHON/bin:$PATH
           .github/workflows/scripts/unix_test.sh
         env:
           TI_WANTED_ARCHS: "metal,vulkan,cpu"
+          PYTHON: ${{ matrix.python }}