Check the build directory for po/ as well

Summary:
Instead of just checking if it's in the source directory, check in the
build directory as well, as ECM can download po/ there as well.

Test Plan: Built KAlgebra translations using the Android docker SDK

Reviewers: Frameworks, ltoscano, dfaure

Reviewed By: ltoscano, dfaure

Subscribers: dfaure, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14572