bug#53406: union-build incorrectly handles grafts

[John Kehayias]

I haven't had the chance to see what is happening inside union-build yet, but 
did confirm that propagating expat/fixed instead of expat does not have the 
same problem.

I tried with

(define fontconfig-fixed
  (package
    (inherit fontconfig)
    (propagated-inputs
     (modify-inputs (package-propagated-inputs fontconfig)
       (replace "expat" (@@ (gnu packages xml) expat/fixed))))))

and then (fhs-union (list fontconfig-fixed)) did not have any broken links 
trying to go to the wrong version. Using this also works around my original 
problem of a multi-arch union build having collisions (so that seems to stem 
from whatever is happening here).