[ci]: Support code diff coverage (sonic-net#934)

Author: xumia

.azure-pipelines/build-template.yml

@@ -41,6 +41,10 @@ jobs:
 - job:
   displayName: ${{ parameters.arch }}
   timeoutInMinutes: ${{ parameters.timeout }}
+  variables:
+    DIFF_COVER_CHECK_THRESHOLD: 0
+    ${{ if eq(parameters.run_unit_test, true) }}:
+      DIFF_COVER_ENABLE: 'true'
 
   pool:
     ${{ if ne(parameters.pool, 'default') }}: