post-build: Check out PR commit as well as trunk for plugin upgrade test

[anomiex]

Proposed changes:

There was a subtle change in the plugin upgrade test in the move from the Build to the Post-Build workflow: since the latter runs via workflow_run, the "current" branch is trunk rather than the PR commit, so that's what gets checked out.

Mostly that works fine, but at one point we look in the checkout for the plugin's wp-plugin-slug. For that one check, we need a copy of the PR commit rather than trunk. But we want to keep everything else running off of trunk to avoid potential security issues (the same ones that led GitHub to introduce the "pull_request_target" event).

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

#28479 (comment)

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• I guess merge this in a forked repo, then make a copy of Add plugin jetpack-mu-wpcom-plugin #28479 in that repo and see if it works.

[github-actions]

Thank you for your PR!

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[samiff]

Let's give it a shot