Backport: Skip pull_request triggered runs in private repos - 4.8 Branch

[desrosj]

This backports 15c4b04 (r63183) to the 4.8 branch.

Merge Conflict Resolution

git cherry-pick was aborted because the vast majority of the workflow files touched by the
original commit do not exist on the 4.8 branch, which only has four workflow files at all:
coding-standards.yml, javascript-tests.yml, phpunit-tests.yml, and test-build-processes.yml.
The change was applied by hand to those four files instead, reusing the original commit message.

• Skipped entirely (file does not exist on 4.8): end-to-end-tests.yml,
  javascript-type-checking.yml, performance.yml, php-compatibility.yml,
  phpstan-static-analysis.yml, test-and-zip-default-themes.yml,
  upgrade-develop-testing.yml, workflow-lint.yml. None of these workflows, or an equivalent
  of them, exist in the 4.8 branch, so there was nothing to update.
• coding-standards.yml: updated the jshint job's if: condition to the new
  private-repo/draft/label-gated form. The slack-notifications and failed-workflow jobs are
  gated on github.event_name != 'pull_request' and were left untouched, matching the original
  commit's scope.
• javascript-tests.yml: updated the test-js job's if: condition the same way. The
  slack-notifications and failed-workflow jobs were left untouched for the same reason as above.
• phpunit-tests.yml: updated the test-php job's if: condition the same way. On this
  branch, test-php only uses the simple
  github.repository == 'WordPress/wordpress-develop' || github.event_name == 'pull_request'
  gate (the startsWith( github.repository, 'WordPress/' ) && (...) wrapper seen in trunk's
  phpunit-tests.yml for its matrix jobs does not exist on 4.8, since this branch predates the
  per-PHP-version reusable-workflow matrix split), so the canonical replacement condition was
  used as-is. The slack-notifications and failed-workflow jobs were left untouched.
• test-build-processes.yml: updated the test-core-build-process job's if: condition the
  same way. The test-core-build-process-macos job uses a stricter, unrelated condition
  (github.repository == 'WordPress/wordpress-develop', with no pull_request allowance at all,
  to limit consumption of the higher-cost macOS runner minutes) and was intentionally left
  untouched, since it is not a member of the ... || github.event_name == 'pull_request' family
  the original commit targeted. The slack-notifications and failed-workflow jobs were left
  untouched as well.

No files outside .github/workflows/ were touched, and no file was created or deleted.

Use of AI Tools

This pull request was created by an AI agent (Claude Code). Until this PR is marked "Ready for Review", treat it as untrusted, AI-created code that requires a manual review by a human team member.

[github-actions]

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