Pushed a potential fix, assuming there's something weird going on with relying on cwd rather than full paths (I know there's some weirdness here when paths get too long, for example).

Zach - this is one of your buildbots. Can we trigger a run from my branch? (As an aside, https://buildbot.python.org/all/#/builders?tags=custom.unstable&tags=custom.stable is currently showing no builders, but I'm not sure we have a custom one that matches the configuration here anyway.)