Build/Test Tools: Don't hardcode the directory name of the repo

When Travis builds the directory, it was looking for a hardcoded `wordpress-develop` directory. If the repo is sitting in another directory, the memcache install will fail. This allows a dynamic path for the directory.

Fixes #48444.
Props pento, whyisjake.

Built from https://develop.svn.wordpress.org/trunk@46605


git-svn-id: https://core.svn.wordpress.org/trunk@46402 1a063a9b-81f0-0310-95a4-ce76da25c4cd