stdenv substitute: avoid using a temporary file

- IMO using a temporary is not needed here (anymore),
- temporary at that location can cause a problem (in a specific case):
for example, when using the substituteAll function from nixpkgs
on a single file directly under /nix/store/ (or ./foo-file),
the stdenv's substitute tries to create a temporary directly under
/nix/store, which causes problems on chrooted darwin
(according to @copumpkin earlier today on IRC)