This line has been this way since it was written, in 9e08f5efe
in 2006.
I think it was just a small mistake then; Eelco's thesis earlier
that year says the permission on each file is set to 0444 or 0555
in a derivation's output as part of the build process. In any
case I'm pretty sure that's the behavior now.
e40e01c1dd