stdenv-darwin: Fix dependency on bootstrapTools

Commit 2040a9ac changed the order of
$PATH elements, causing initialpath to appear after buildInputs. Thus
gnugrep ended up depending on bin/sh from bootstrapTools, rather than
from pkgs.bash. The fix is to provide pkgs.bash via buildInputs rather
than initialPath.

http://hydra.nixos.org/build/33276697