[tlbuild] build with non-standard TL_WORKDIR fails in luatexdir

[Karl Berry]

    is building outside the (read-only) source tree supposed to work? I

Sure.

    just tried and it failed in luatexdir:
    
I did not get this problem with the usual Build where the Work dir is a
subdir of source.  How exactly are you setting things up, ie, what
should I do to try to reproduce?

(I can easily believe there is such a problem.)

Thanks,
k