Build/Test Tools: Skip copying `*.min.asset.php` files for scripts.

The `index.min.asset.php` file for each script bundled through the built asset from the `gutenberg` repository contains the `dependencies` and `version` hash for each script.

These values are merged into the `script-loader-packages.php` file during a `build`, so the original PHP files are not required.

Props peterwilsoncc, desrosj.
Fixes #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@62073 602fd350-edb4-49c9-b593-d223f7449a82