fix(browser): set 'BROWSER' environment variable based on nix config

home/modules/browser/default.nix

@@ -1,4 +1,4 @@
-{ lib, ... }:
+{ config, lib, ... }:
 
 {
   options.browser = {
@@ -23,6 +23,8 @@
     };
   };
 
+  config.home.sessionVariables.BROWSER = config.browser.primary;
+
   imports = [
     ./firefox.nix
     ./librewolf.nix