Build SVN access for SergeyBiryukov

When WordPress is in RC stage and a new version of Akismet is released, the svn:externals property has to be updated on the build repo (core.svn.wordpress.org).

For the upcoming WordPress 5.4 release this is already handled in https://core.trac.wordpress.org/ticket/49662#comment:2, so we’re good for now 🙂

Would it be possible for me (SergeyBiryukov) to get write access to core.svn.wordpress.org so I could help with these tasks in the future?

#prio3