build-support: Fix nix-prefetch-* on OS X.

Fixes a regression on OS X introduced by f83af95.

Don't use --tmpdir for mktemp, because that flag doesn't exist on OS X.
However, using -t is deprecated in GNU coreutils, so as suggested by
@ip1981 we're now using parameter expansion on ${TMPDIR:-/tmp} to
provide /tmp as a fallback if TMPDIR is not set and use it instead.

Also use this approach for nix-prefetch-cvs now in order to stay
consistent.

Reported-by: Vladimir Kirillov <proger@wilab.org.ua>
Tested-by: Igor Pashev <pashev.igor@gmail.com>
Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -256,7 +256,7 @@ else
   # download the file and add it to the store.
   if test -z "$finalPath"; then
 
-      tmpPath="$(mktemp --tmpdir -d git-checkout-tmp-XXXXXXXX)"
+      tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/git-checkout-tmp-XXXXXXXX")"
       trap "rm -rf \"$tmpPath\"" EXIT
 
       tmpFile="$tmpPath/git-export"