How to only run build on PR?

[littleGnAl]

I want to run the build on PR only and filter specific base branches as my github action does:

on:
  pull_request:
    branches: 
      - main
      - release/**
      - special/**
      - dev/**

I tried to add only_if expression to strict the build to run on PR only, but it seems not to work correctly, the PR skipped but with the correct config. Here's my implementation, can you help me check the logic?
https://github.com/AgoraIO-Extensions/Agora-Flutter-SDK/blob/0d13ec68f1f8198c26ebc24be2ad4213da2b1dd8/.cirrus.yml#L7

[fkorotkov]

I think you should check for CIRRUS_BRANCH instead of CIRRUS_BASE_BRANCH which is used for PRs.