fix appvayor.yml

appveyor.yml

@@ -78,7 +78,7 @@ install:
   # pip will build them from source using the MSVC compiler matching the
   # target Python version and architecture
   - "%CMD_IN_ENV% pip install -r dev-requirements.txt"
-  - "IF %UIA_SUPPORT% == \"YES\" pip install comtypes"
+  - "IF %UIA_SUPPORT% == \"YES\" %CMD_IN_ENV% pip install comtypes"
 
   # Enable desktop (for correct screenshots).
   #- ps: iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-desktop.ps1'))