fix(builder): exec the runner
fix(builder): Non-root slugrunner and slugbuilder

This improves the compatibility to heroku and stops running apps as root
fix(builder): Reverted to default php buildpack

The pervious compatibility fixes made the heroku php build pack work
out-of-the-box again
ref(tests): update details in CI node setup script
Merge pull request #1983 from mboersma/test-setup-tweaks

ref(tests): update details in CI node setup script
Merge pull request #1972 from johanneswuerbach/buildpack-env

fix(builder): Non-root slugrunner and slugbuilder