Update appveyor config to newer appveyor-rust scheme.

Author: AltSysrq

appveyor.yml

@@ -1,12 +1,15 @@
 environment:
   matrix:
-  - TARGET: nightly-x86_64-pc-windows-msvc
-  - TARGET: nightly-x86_64-pc-windows-gnu
-  - TARGET: 1.50.0-x86_64-pc-windows-gnu
+  - channel: nightly
+    target: x86_64-pc-windows-msvc
+  - channel: nightly
+    target: x86_64-pc-windows-gnu
+  - channel: 1.50.0
+    target: x86_64-pc-windows-gnu
 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"
+  - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
+  - rustup-init -yv --default-toolchain %channel% --default-host %target%
+  - set PATH=%PATH%;%USERPROFILE%\.cargo\bin
   - rustc -vV
   - cargo -vV
 build_script: