ci: install pnpm globally in inside ci

Author: aminya

script/vsts/platforms/templates/preparation.yml

@@ -28,12 +28,17 @@ steps:
       force32bit: $(IsWinX86)
     displayName: Install Node.js 12.18.3
 
-  - script: npm install --global npm@7
-    displayName: Update npm
+  - script: |
+      # install npm
+      npm install --global npm@7
+
+      # install pnpm
+      npm install --global pnpm@6
+
+      # papm uses this. TODO use npx instead of directly calling shx
+      npm install --global shx
 
-  # papm uses this. TODO use npx instead of directly calling shx
-  - script: npm install --global shx
-    displayName: Install shx
+    displayName: Install npm, pnpm, shx
 
   - pwsh: |
       cd script/vsts