How to change environment variables for package

I have a package that now requires a third party ppa which installs its libraries into the /opt/qt57 directory. The new ppa provides a shell script to setup the correct environment variables when I build locally, but I don't know how to make this happen when generating the package.source.changes file that gets uploaded to Launchpad.net. I have read that maybe it should be done in the debian/rules file but not sure what to change or add. The source that I am creating the Debian for can be found here https://github.com/Levi-Armstrong/qtermwidget .