Update Rust installer task

Author: alexcrichton

ci/azure-install-rust.yml

@@ -5,15 +5,17 @@ steps:
       if [ "$toolchain" = "" ]; then
         toolchain=stable
       fi
-      curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain $toolchain
-      echo "##vso[task.prependpath]$HOME/.cargo/bin"
+      if command -v rustup; then
+        rustup update --no-self-update $toolchain
+        rustup default $toolchain
+      else
+        curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain $toolchain
+        echo "##vso[task.prependpath]$HOME/.cargo/bin"
+      fi
     displayName: Install rust (unix)
     condition: ne( variables['Agent.OS'], 'Windows_NT' )
 
-  - script: |
-      curl -sSf -o rustup-init.exe https://win.rustup.rs
-      rustup-init.exe -y --default-toolchain stable-%TARGET%
-      echo ##vso[task.prependpath]%USERPROFILE%\.cargo\bin
+  - bash: rustup update --no-self-update stable-$TARGET && rustup default stable-$TARGET
     displayName: Install rust (windows)
     condition: eq( variables['Agent.OS'], 'Windows_NT' )