message: add cargo cache for appveyor
diff: @@ -12,6 +12,9 @@ environment:
  ABI: msvc
  TARGET: x86_64-pc-windows-msvc
  +cache:
  + - 'C:\Users\appveyor\.cargo'
  +
  install:
  - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
  - rustup-init.exe -yv --default-host %target%
  