Build doesn't run with Forked repo's PR

[KOUNOIKE Yuusuke]

Steps to reproduce:

1. create repo (root/test)
2. commit some changes
3. fork repo by other user (user/test)
4. create branch (pr) on user/test
5. commit some changes to pr
6. create PR from user/test to root/test

expected: build run
actual: doesn't run

7. some additional commit on pr branch and push it

expected: build run
actual: doesn't run

I think to running build at 7. additional hook point required.