msbuild dogfood, pipeline modification attempt no 1.

Author: SimaTian

.azure/pipelines/ci-public.yml

@@ -77,6 +77,8 @@ variables:
   value: false
 - name: Codeql.SkipTaskAutoInjection
   value: true
+- name: MSBUILDUSESERVER
+  value: 1
 - template: /eng/common/templates/variables/pool-providers.yml
 
 stages: