Since the release of version 0.10, which properly fixes the issue in specifying that moduleroot should be default directory, I can no longer specify a custom phing build file path.
looking for '/var/lib/jenkins/jobs/[SOME_JOB]/workspace/Build/Scripts/build.xml' ...
[workspace] $ phing -buildfile build.xml build-all deploy [SOME_PROPERTIES_WERE_OMITTED]
Buildfile: build.xml does not exist!
This absolute path does actually exist, and 'Phing Build File' path is simply 'Build/Scripts/build.xml'. So it looks like it is looking for the file in the right location, but then reverts to using just the filename when executing phing.