task: Make the build name public

We'll use this instead of the CWD when reporting failures
task: Make the CWD public as well

The make builtin needs this to see the output of a task.
autobuilder: Don't bother passing a blank onEmpty

Just check for not-null when we call it.
task: Don't bother reporting success/error in onEmpty

It doesn't work anyway, and isn't really an API we can rely on.
task: Save the buildPath on the runner
autobuilder: Add a convenient set of symlinks for the last successful/failed build