Build: Faster Gutenberg Builds

[youknowriad]

This updates the Gutenberg build integration to use the new --fast flag, which skips TypeScript-related steps (version validation, tsc --build, and type declaration checks) that aren't needed when building for WordPress Core. These steps only produce .d.ts type declaration files which aren't shipped with Core.

The Gutenberg ref is also updated to include the commit that adds the --fast flag support.

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

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Props youknowriad.

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[github-actions]

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The changes in this pull request can previewed and tested using a WordPress Playground instance.

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• The Plugin and Theme Directories cannot be accessed within Playground.
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For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[youknowriad]

For reference, I compared the build step in three jobs:

1- Before the move to the new integration flow 1m19 seconds
2- After the move to the new integration flow 6m17 seconds
3- After this PR 1m49 seconds

So I think with this PR we're effectively back to similar decent performance levels for the build commands and comparable to previous jobs without losing the benefits for the "checkout & build" approach.