nixos/sway: Extend the default configuration for NixOS

The default config.in template contains
"include @sysconfdir@/sway/config.d/*" but we've dropped it to better
support non-NixOS (which seems like a mistake in retrospect).
This restores that behaviour and extends the default configuration via
nixos.conf to fix #119445.

Note: The security configurations (security.d) where dropped entirely
(but maybe they'll return).

pkgs/applications/window-managers/sway/default.nix

@@ -4,6 +4,8 @@
 , pango, cairo, libinput, libcap, pam, gdk-pixbuf, librsvg
 , wlroots, wayland-protocols, libdrm
 , nixosTests
+# Used by the NixOS module:
+, isNixOS ? false
 }:
 
 stdenv.mkDerivation rec {
@@ -27,6 +29,10 @@ stdenv.mkDerivation rec {
     })
   ];
 
+  postPatch = lib.optionalString isNixOS ''
+    echo -e '\ninclude /etc/sway/config.d/*' >> config.in
+  '';
+
   nativeBuildInputs = [
     meson ninja pkg-config wayland scdoc
   ];