CI: Skip workflow jobs that should not run on forks

[dd32]

What

Adds fork-repository guards to three workflow jobs that currently run (and in one case, fail) on every push to trunk in forked repositories.

Why

When syncing a fork of WordPress/gutenberg, every push to trunk triggers workflow runs. Most jobs are correctly gated with github.repository == 'WordPress/gutenberg' || github.event_name == 'pull_request', but three are not:

1. unit-test.yml → unit-php aggregator job — gated only by if: always(). When test-php and phpcs are skipped on a fork, this job runs and evaluates needs.test-php.result != 'success'. Since skipped != success, it exits 1, marking every fork push as a failed Unit Tests run. This is the most visible problem — forks see a red ❌ on every commit synced from upstream.

2. end2end-test.yml → merge-artifacts job — gated only by if: !cancelled(). The upstream e2e-playwright job is correctly skipped on forks, but merge-artifacts still spins up a runner, checks out the repo, and runs npm install for nothing.

3. sync-backport-changelog.yml — has no fork guard at all. On every push to trunk in a fork it attempts to look up an issue with the 🤖 Sync Backport Changelog label in the fork itself, which is meaningless outside of WordPress/gutenberg.

How

Each job now also checks github.repository == 'WordPress/gutenberg' || github.event_name == 'pull_request', matching the guard already used by the sibling jobs in the same workflows.

Testing instructions

• Verify the unit-test workflow still passes on a PR (the pull_request branch of the condition is unchanged).
• After merging, fork pushes to trunk should no longer show the failing Unit Tests run, the Merge Artifacts job should be skipped, and Sync Core Backport Issue should not run.

🤖 Generated with Claude Code

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[dd32]

The above is Claude-generated.

The reason I was looking into this was that my personal free GitHub actions usage/actions-storage was being used up in a matter of days, Gutenberg (behind Playground) was the largest user of resources.