add if

Author: Tieqiong

.github/workflows/osx64.yml

@@ -38,8 +38,9 @@ jobs:
 
       - name: Install diffpy.pdfgui and requirements
         run: |
-          export CONDA_OVERRIDE_OSX=11.0
-          conda info
+          if [[ "${{ matrix.os }}" == "macos-13" ]]; then
+            export CONDA_OVERRIDE_OSX=11.0
+          fi
           conda install --file requirements/test.txt
           conda install wxpython diffpy.utils matplotlib-base
           pip install diffpy.pdffit2==1.5.0rc1