build: use pull_request_target to start Jenkins
GitHub recently introduced a new Pull Request event for Actions with access to Secrets and GITHUB_TOKEN with write access. Therefore, we don't need to use the scheduler event to start Jenkins anymore.
Ref: https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines