Build/Test Tools: Properly transform `theme.json` files during copy.

The `theme.json` file `$schema` URL is relative in the `gutenberg` repository upstream. The URL is not currently being replaced with an aboslute one pointing to w.org due to a missing `transform: true` configuration.

Follow up to [61438], [61439], [61458], [61492], [61677].

See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@61867 602fd350-edb4-49c9-b593-d223f7449a82