fix snap build

build/azure-pipelines/linux/snap-build-linux.yml

@@ -19,6 +19,11 @@ steps:
       sudo apt-get update
       sudo apt-get upgrade -y
 
+      # Yarn
+      curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
+      echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
+      apt-get update && apt-get install -y yarn
+
       # Define variables
       REPO="$(pwd)"
       SNAP_ROOT="$REPO/.build/linux/snap/$(VSCODE_ARCH)"