ci(appveyor): prevent duplicate builds

Skip feature branches with open PR.

See https://github.com/appveyor/ci/issues/882

appveyor.yml

@@ -25,6 +25,8 @@ shallow_clone: true
 
 clone_depth: 1
 
+skip_branch_with_pr: true
+
 install:
   - ps: Install-Product node $env:nodejs_version
   - set CI=true