Surround rustc download in appveyor-retry

Author: alexcrichton

appveyor.yml

@@ -7,7 +7,7 @@ environment:
   - TARGET: x86_64-pc-windows-msvc
   - TARGET: i686-pc-windows-msvc
 install:
-  - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
+  - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init.exe -y --default-host %TARGET%
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
   - if defined MSYS2_BITS set PATH=%PATH%;C:\msys64\mingw%MSYS2_BITS%\bin