Match Scala version for 2.13.x in CI
Match Scala version for 2.13.x in CI

Problem / Solution

We recently upgraded scala to 2.13.6 but neglected to update the version for Github Actions.
This PR fixes that.

Signed-off-by: Ruben Oanta <roanta@twitter.com>

Differential Revision: https://phabricator.twitter.biz/D691500

twitter/util,twitter/util,twitter/util