ci: add npm revert step to azure

Author: phated

.ci/.azure-pipelines-steps.yml

@@ -4,6 +4,9 @@ steps:
     versionSpec: '$(node_version)'
   displayName: Use Node $(node_version)
 
+- script: npm install -g [email protected]
+  displayName: Revert npm to 6.4.1 (known working)
+
 - script: npm install
   displayName: npm install