Update appveyor.yml

appveyor.yml

@@ -46,6 +46,7 @@ environment:
     
 install:
   - if [%COMPILER%]==[clang-cl] call %CONDA_INSTALL_LOCN%\Scripts\activate.bat
+  - if [%COMPILER%]==[clang-cl] conda update -n base conda
   - if [%COMPILER%]==[clang-cl] conda config --add channels conda-forge --force
   - if [%COMPILER%]==[clang-cl] conda config --set auto_update_conda false
   - if [%COMPILER%]==[clang-cl] conda install --yes --quiet clangdev cmake ninja flang=11.0.1