Build/Test Tools: Exclude plugins and themes within `src`.

Just like the `build` directory, `src` can include plugins and themes which shouldn't be linted by PHPCS.

Fixes #49781
Built from https://develop.svn.wordpress.org/trunk@47742


git-svn-id: https://core.svn.wordpress.org/trunk@47518 1a063a9b-81f0-0310-95a4-ce76da25c4cd