sway: add preCheckConfig to customize check environment

See e.g. https://github.com/nix-community/home-manager/issues/5311

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

@@ -269,6 +269,7 @@ let
     checkPhase = lib.optionalString cfg.checkConfig ''
       export DBUS_SESSION_BUS_ADDRESS=/dev/null
       export XDG_RUNTIME_DIR=$(mktemp -d)
+      ${cfg.preCheckConfig}
       ${pkgs.xvfb-run}/bin/xvfb-run ${swayPackage}/bin/sway --config "$target" --validate --unsupported-gpu
     '';
 
@@ -478,6 +479,19 @@ in {
       description = "Sway configuration options.";
     };
 
+    preCheckConfig = mkOption {
+      type = types.lines;
+      default = "";
+      example = ''
+        export HOME=$(mktemp -d)
+        touch ~/background.png
+      '';
+      description = ''
+        Script to run before running `sway --validate` on the generated config
+        file. Only used if `checkConfig = true`.
+      '';
+    };
+
     checkConfig = mkOption {
       type = types.bool;
       default = true;

tests/modules/services/window-managers/sway/default.nix

@@ -6,6 +6,7 @@
   sway-followmouse = ./sway-followmouse.nix;
   sway-followmouse-legacy = ./sway-followmouse-legacy.nix;
   sway-check-config = ./sway-check-config.nix;
+  sway-check-config-precheck = ./sway-check-config-precheck.nix;
   sway-modules = ./sway-modules.nix;
   sway-no-xwayland = ./sway-no-xwayland.nix;
   sway-null-config = ./sway-null-config.nix;

tests/modules/services/window-managers/sway/sway-check-config-precheck.nix

@@ -0,0 +1,17 @@
+{ config, lib, ... }:
+
+lib.mkIf config.test.enableBig {
+  wayland.windowManager.sway = {
+    enable = true;
+    checkConfig = true;
+    preCheckConfig = ''
+      export HOME=$(mktemp -d)
+      touch ~/mybg.png
+    '';
+    config.output."*".background = "~/mybg.png fill";
+  };
+
+  nmt.script = ''
+    assertFileExists home-files/.config/sway/config
+  '';
+}