Add input to the test Gutenberg build workflow

[desrosj]

This adds a new input for the reusable workflow responsible for testing the Gutenberg build script to allow branches to use a specific branch in the gutenberg repository.

This allows for a similar pattern to that used in the end to end testing workflow where a specific version of the Gutenberg plugin is pinned once support for the branch of WordPress running the workflow is dropped.

The motive for this change came about from this failed workflow run, which started failing after https://github.com/wordpress/gutenberg was updated to use Node.js version 24.x instead of 20.x, which is what the 6.8 branch uses.

Trac ticket: Core-66040.

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[Copilot]

🟢 Approval recommended

The change is small, self-contained, and consistent with existing workflow input patterns while addressing the stated CI compatibility need.

Pull request overview

This PR updates the reusable GitHub Actions workflow that validates the Gutenberg plugin build process inside a wordpress-develop checkout by allowing callers to pin the Gutenberg repository checkout to a specific branch. This enables older WordPress branches (e.g., ones still using Node.js 20.x) to run the workflow against a compatible Gutenberg branch after upstream changes (e.g., Node.js 24.x) land on trunk.

Changes:

• Adds a new workflow_call input (gutenberg-branch) to control which Gutenberg branch is checked out.
• Updates the Gutenberg checkout step to use that input (falling back to trunk when not provided).

File summaries

File	Description
.github/workflows/reusable-test-gutenberg-build-process.yml	Adds an optional input to pin the Gutenberg checkout ref, defaulting to trunk.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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[lancewillett]

Code and approach look OK to land.

• The input defaults safely to trunk; linked branch CI verified pinned Gutenberg branches across Linux, macOS, and Windows
• The original failure is a deterministic Node/Gutenberg dependency mismatch