Build: Fix Gutenberg copy script missing constants.php and incorrect base URL

[youknowriad]

Summary

The previous Gutenberg hash upgrade brought a new @wordpress/build package to Core but the build scripts in Core were not updated in consequence. It resulted in a broken font library page. This PR fixes it.

• Add constants.php to the list of PHP infrastructure files copied from Gutenberg's build output to Core in copy-gutenberg-build.js
• Fix the --base-url argument in build-gutenberg.js to include the trailing slash inside includes_url('build/') instead of outside the function call
• Rename fast to skip-types to get back to sane performance for CI jobs and gutenberg builds.

Fixes https://core.trac.wordpress.org/ticket/64656.

Test plan

• Visit the Font Library admin page and confirm it loads without errors

🤖 Generated with Claude Code

[github-actions]

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Props youknowriad.

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[github-actions]

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[ellatrix]

Thanks, Font Library works now.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61673
GitHub commit: 5439957

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.