Code review comment for ~cjwatson/launchpad-buildd:ci-chown-build-tree

I've going to self-approve this - it's a regression in launchpad-buildd 227 that affected Launchpad's own CI jobs, and I have a simple end-to-end test case for it on dogfood that I'll be able to run easily (https://code.dogfood.paddev.net/~cjwatson/+git/git-test).