ci(build.yml): add pnpm installation to azure pipeline (#966)

I used this https://pnpm.io/continuous-integration#azure-pipelines snippet from official site.

jobs/build.yml

@@ -24,6 +24,10 @@ jobs:
       versionSpec: $(node_version)
   - script: git config --global user.email "[email protected]"
   - script: git config --global user.name "Example Git User"
+  - script: |
+      curl -f https://get.pnpm.io/v6.16.js | node - add --global pnpm@6
+      pnpm config set store-dir $(pnpm_config_cache)
+    displayName: "Setup pnpm"
   - script: npm install
   - script: npm run build
   - script: npm test && npm run write-coverage
@@ -42,4 +46,4 @@ jobs:
     displayName: Publish packages
     condition: and(succeeded(), eq(variables['system.pullrequest.isfork'], false))
     inputs:
-      pathtoPublish: '$(Build.ArtifactStagingDirectory)'
+      pathtoPublish: '$(Build.ArtifactStagingDirectory)'