Update Rust install task

Author: alexcrichton

libm/ci/azure-install-rust.yml

@@ -10,7 +10,7 @@ steps:
         rustup default $toolchain
       else
         curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain $toolchain
-        echo "##vso[task.setvariable variable=PATH;]$PATH:$HOME/.cargo/bin"
+        echo "##vso[task.prependpath]$HOME/.cargo/bin"
       fi
     displayName: Install rust (unix)
     condition: ne( variables['Agent.OS'], 'Windows_NT' )