properly call the rust installer to not require user input

appveyor.yml

@@ -5,7 +5,7 @@ environment:
 
 install:
   - ps: Start-FileDownload "https://static.rust-lang.org/dist/rust-nightly-${env:TARGET}.exe"
-  - rust-nightly-%TARGET%.exe
+  - rust-nightly-%TARGET%.exe /VERYSILENT /NORESTART /DIR="C:\Program Files (x86)\Rust"
   - SET PATH=%PATH%;C:\Program Files (x86)\Rust\bin
   - rustc -Vv
   - cargo -V