Skip AI Chat service key .env assert for Brave Origin builds (#36961)

Brave Origin builds disable AI Chat (enable_ai_chat =
!is_brave_origin_branded), so service_key_aichat is unused and should not
be required in .env.

components/ai_chat/core/common/buildflags/BUILD.gn

@@ -6,6 +6,7 @@
 # TODO(https://github.com/brave/brave-browser/issues/48186): Move to root of
 # component.
 
+import("//brave/components/brave_origin/buildflags/buildflags.gni")
 import("//build/buildflag_header.gni")
 import("buildflags.gni")
 
@@ -14,7 +15,11 @@ declare_args() {
 }
 
 if (is_official_build) {
-  assert(service_key_aichat != "")
+  # Not needed for Brave Origin builds, which ship with AI Chat disabled
+  # (enable_ai_chat = !is_brave_origin_branded).
+  if (!is_brave_origin_branded) {
+    assert(service_key_aichat != "")
+  }
 }
 
 buildflag_header("buildflags") {