Build/Test Tools: Only copy block library-related CSS to a single location.

Since [61438], all CSS files for the block library are being copied twice into two separate locations: `wp-includes/blocks/BLOCKNAME/` (the previous location) and `wp-includes/css/dist/block-library/BLOCKNAME/`.

Since the copies added to the `css/dist` directory are not actually being used and moving the files was the original intention in [61438], this adds an exclusion rule to the `grunt copy:gutenberg-styles` task to allow the `tools/gutenberg/copy.js` script to handle these files.

Props jorbin, youknowriad.
See #64933.

git-svn-id: https://develop.svn.wordpress.org/trunk@62103 602fd350-edb4-49c9-b593-d223f7449a82