This is consistent with what 'gnu-build' does and makes sure origins
aren't getting lowered with #:graft? #t in one case and not in the
other.
This is a followup to df46bef48eaa43c502fa9193371692c039b460c1 and
dada5e86ed58c1869e30678c4d69582b1528ed6c.
* guix/build-system/python.scm (python-build): Pass #:graft? #f.