sway: add PATH to systemd.variables

Flatpak apps use xdg-desktop-portals to open e.g. links in the default
browser.
For this to work, the xdg-desktop-portal user service needs the PATH
environment variable to be set correctly.
See the following issue and especially the linked comment for details:
https://github.com/NixOS/nixpkgs/issues/189851#issuecomment-1238907955

modules/services/window-managers/i3-sway/sway.nix

@@ -387,6 +387,7 @@ in {
           * {env}`NIXOS_OZONE_WL`
           * {env}`XCURSOR_THEME`
           * {env}`XCURSOR_SIZE`
+          * {env}`PATH`
           You can extend this list using the `systemd.variables` option.
         '';
       };
@@ -402,6 +403,7 @@ in {
           "NIXOS_OZONE_WL"
           "XCURSOR_THEME"
           "XCURSOR_SIZE"
+          "PATH"
         ];
         example = [ "-all" ];
         description = ''