Action raises errors for non PR check runs

[dlackty]

While trying to debug #9 , I set up a sample project and found an issue that this action now only works with pull requests:

/action/lib/github_client.rb:40:in `request_http': Not Found: {"message":"Not Found","documentation_url":"https://docs.github.com/rest"} (RuntimeError)
[236](https://github.com/polydice/railsgoat/runs/6637854982?check_suite_focus=true#step:4:237)
	from /action/lib/github_client.rb:19:in `post'
[237](https://github.com/polydice/railsgoat/runs/6637854982?check_suite_focus=true#step:4:238)
	from /action/lib/github_check_run_service.rb:47:in `client_post_pull_requests'
[238](https://github.com/polydice/railsgoat/runs/6637854982?check_suite_focus=true#step:4:239)
	from /action/lib/github_check_run_service.rb:31:in `block in run'
[239](https://github.com/polydice/railsgoat/runs/6637854982?check_suite_focus=true#step:4:240)
	from /action/lib/github_check_run_service.rb:28:in `each'
[240](https://github.com/polydice/railsgoat/runs/6637854982?check_suite_focus=true#step:4:241)
	from /action/lib/github_check_run_service.rb:28:in `each_slice'
[241](https://github.com/polydice/railsgoat/runs/6637854982?check_suite_focus=true#step:4:242)
	from /action/lib/github_check_run_service.rb:28:in `run'
[242](https://github.com/polydice/railsgoat/runs/6637854982?check_suite_focus=true#step:4:243)
	from /action/lib/index.rb:34:in `<main>'

Specifically, the following code causes the issue:

https://github.com/cookpad/brakeman-linter-action/blob/v2.0.1-rc.1/lib/github_check_run_service.rb#L62-L64

Maybe we should check if this is an action run triggered by PR before calling this GitHub API?