Use mktemp for tempdir creation in prefetchers.

This incorporates the following two commits from <nixpkgs>:

NixOS/nixpkgs@f83af95f8a54d0375ac6e159b663de887ba5b6a6 NixOS/nixpkgs@5e7a1cf955e43e1fa3c1c157c0d6961e1bcf9801

Hydra was the original reason why I was fixing tempdir creation in the first place. Seeing that Hydra ships its own versions of these scripts, we need to patch them here as well.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

Created by  aszlig  on August 28, 2014
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