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/fetchcvs/nix-prefetch-cvs

@@ -20,7 +20,7 @@ fi
 
 
 mkTempDir() {
-    tmpPath=$(mktemp -d -t nix-prefetch-cvs-XXXXXXXX)
+    tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/nix-prefetch-cvs-XXXXXXXX")"
     trap removeTempDir EXIT SIGINT SIGQUIT
 }