Use mktemp for tempdir creation in prefetchers.

This incorporates the following two commits from <nixpkgs>:

NixOS/nixpkgs@f83af95f8a
NixOS/nixpkgs@5e7a1cf955

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>

src/script/nix-prefetch-git

@@ -237,11 +237,11 @@ else
   # download the file and add it to the store.
   if test -z "$finalPath"; then
 
-      tmpPath=/tmp/git-checkout-tmp-$$
-      tmpFile=$tmpPath/git-export
-      mkdir $tmpPath $tmpFile
+      tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/git-checkout-tmp-XXXXXXXX")"
+      trap "rm -rf \"$tmpPath\"" EXIT
 
-      trap "rm -rf $tmpPath" EXIT
+      tmpFile="$tmpPath/git-export"
+      mkdir "$tmpFile"
 
       # Perform the checkout.
       clone_user_rev "$tmpFile" "$url" "$rev"