fix 'hardened' profile for NixOS 20.09

The 'scudo' memory allocator set by the 'hardened' profile breaks some
services on 20.09.
The fix for NixOS unstable (https://github.com/NixOS/nixpkgs/pull/104052)
is ineffective on 20.09.

As a workaround, add a custom 'hardened' preset that uses the default allocator.

modules/presets/hardened.nix

@@ -0,0 +1,14 @@
+{
+  imports = [
+    # Source: https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/profiles/hardened.nix
+    <nixpkgs/nixos/modules/profiles/hardened.nix>
+  ];
+
+  ## Reset some options set by the hardened profile
+
+  # Needed for sandboxed builds and services
+  security.allowUserNamespaces = true;
+
+  # The "scudo" allocator is broken on NixOS 20.09
+  environment.memoryAllocator.provider = "libc";
+}