appveyor: on_finish run after saving cache

Use after_test instead

Author: tesuji

appveyor.yml

@@ -15,7 +15,7 @@ branches:
 
 cache:
     - '%USERPROFILE%\.cargo'
-on_finish:
+after_test:
     - cargo install -Z install-upgrade cargo-cache --debug
     - cargo cache --autoclean