Update/move gutenberg checkout to postinstall

[youknowriad]

Trac ticket: https://core.trac.wordpress.org/ticket/64393

Moves Gutenberg checkout to postinstall hook, separating dependency setup from build. Removes checkout step from the build step.

Benefits:

• Faster build times: checkout happens once at install, not every build (although the script was already accounting for this before)
• Cleaner separation of concerns: dependencies are ready before any build runs

Test plan

• Run npm install and verify Gutenberg checkout runs automatically
• Run npm run build and verify build completes successfully
• Run npm run build:dev and verify dev build works

[github-actions]

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Props youknowriad.

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[github-actions]

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[youknowriad]

Noting that with this PR the "build" step of WordPress takes 30 seconds less than what we used to have before the change in Gutenberg integration. The tradeoff is that the "install" step is a little bit slower. This feels like a win since building is something you do more often and install is done once.

[youknowriad]

Commtted in r61458.