ci: use ##vso[task.prependpath] on azure

Author: pietroalbini

.azure-pipelines/steps/linux.yml

@@ -12,7 +12,9 @@ steps:
     chmod +x $HOME/stamp
 
     export PATH=$PATH:$HOME/.local/bin:$HOME/Library/Python/2.7/bin/:$HOME
-    echo "##vso[task.setvariable variable=PATH;]$PATH"
+    echo "##vso[task.prependpath]$HOME/.local/bin"
+    echo "##vso[task.prependpath]$HOME/Library/Python/2.7/bin"
+    echo "##vso[task.prependpath]$HOME"
 
     mkdir -p $HOME/rustsrc
   displayName:  Prep