Add pull_request_target to prEvents

[jurijzahn8019]

Hi there!

i am using pull_request_target as trigger in some workflows. Since this is basically same trigger as the pull_request trigger, they should be handled similary.

https://github.com/LouisBrunner/checks-action/blob/3d24d4813a797720cc4e2080a50bdafb3373aef1/src/main.ts#L13-L16

Thanks!