Do not build Azure Pipelines on Pull Request

For 2 reasons:

* bundled_gems tend to be left broken (like now)
* We don't want Slack notification for pull request and it can't be
  disabled on Azure Pipelines

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67273 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

azure-pipelines.yml

@@ -1,3 +1,4 @@
+pr: none # comment out this line if you need to test this file on PR
 jobs:
 - job: Linux
   pool: