Use rustup

appveyor.yml

 environment:
-  TARGET: nightly-x86_64-pc-windows-gnu
+  TARGET: nightly-x86_64-pc-windows-msvc
 install:
-  - ps: Start-FileDownload "https://static.rust-lang.org/dist/rust-${env:TARGET}.exe" -FileName "rust-install.exe"
-  - ps: .\rust-install.exe /VERYSILENT /NORESTART /DIR="C:\rust" | Out-Null
-  - ps: $env:PATH="$env:PATH;C:\rust\bin"
+  - curl -sSf -o rustup-init.exe https://win.rustup.rs/
+  - rustup-init.exe -y --default-host %TARGET%
+  - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
   - rustc -V
   - cargo -V