re-add libsandbox

This was accidentially reverted in the previous PR.
Interestingly it seems to build without it?
Anyway. It's probably better to have it explicitly in rather than
relying in implicit behaviour of the current sdk.

pkgs/tools/package-management/nix/common.nix

@@ -16,6 +16,7 @@ let
   atLeast210 = lib.versionAtLeast version "2.10pre";
   atLeast213 = lib.versionAtLeast version "2.13pre";
   atLeast214 = lib.versionAtLeast version "2.14pre";
+  atLeast218 = lib.versionAtLeast version "2.18pre";
   atLeast219 = lib.versionAtLeast version "2.19pre";
   atLeast220 = lib.versionAtLeast version "2.20pre";
   atLeast221 = lib.versionAtLeast version "2.21pre";
@@ -42,6 +43,7 @@ in
 , callPackage
 , coreutils
 , curl
+, darwin
 , docbook_xsl_ns
 , docbook5
 , editline
@@ -158,6 +160,8 @@ self = stdenv.mkDerivation {
     libseccomp
   ] ++ lib.optionals withAWS [
     aws-sdk-cpp
+  ] ++ lib.optional (atLeast218 && stdenv.hostPlatform.isDarwin) [
+    darwin.apple_sdk.libs.sandbox
   ];