Fix some bash syntax

Author: alexcrichton

ci/azure-install-rust.yml

@@ -26,7 +26,7 @@ steps:
         rustup default $toolchain
       else
         curl.exe -sSf -o rustup-init.exe https://win.rustup.rs
-        ./rustup-init.exe -y --default-toolchain $env:TOOLCHAIN
+        ./rustup-init.exe -y --default-toolchain $toolchain
         echo "##vso[task.prependpath]$USERPROFILE/.cargo/bin"
       fi
     displayName: Install rust (windows)