pr-publish: make it work on linux too

[dawidd6]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

With this patch it becomes possible to successfully use pr-publish command on linux to dispatch the event in homebrew-core repo.

cc @jonchang

[jonchang]

Looks good!

[MikeMcQuaid]

Thanks @dawidd6!