Build/Test Tools: Add 7.0-RC2 to upgrade testing workflows.

Since this workflow tests based on tagged versions available through WP-CLI, the pre-release versions need to be used.

Follow up to [62160].

See #64966.

git-svn-id: https://develop.svn.wordpress.org/trunk@62166 602fd350-edb4-49c9-b593-d223f7449a82