Engineering - fix CLI build for Alpine ARM64 (microsoft#184309)

* Add task to install yarn

* Move task

* Add condition

Author: lszomoru

build/azure-pipelines/alpine/cli-build-alpine.yml

@@ -16,6 +16,14 @@ steps:
   - ${{ if ne(parameters.VSCODE_QUALITY, 'oss') }}:
     - template: ../distro/download-distro.yml
 
+    # Install yarn as the ARM64 build agent is using vanilla Ubuntu
+    - ${{ if eq(parameters.VSCODE_BUILD_ALPINE_ARM64, true) }}:
+      - task: Npm@1
+        displayName: Install yarn
+        inputs:
+          command: custom
+          customCommand: install --global yarn
+
     - script: |
         set -e
         yarn --frozen-lockfile --ignore-optional