Skip the aggregate Rust library probe for Gentoo system-library builds

Gentoo links the separately packaged tlottie and wallet_engine libraries,
so do not search for the aggregate tdesktop_rust library. This also avoids
the ebuild's find_library(... REQUIRED) rewrite making the probe fatal.

Matches Gentoo's empty-module approach:
https://github.com/gentoo/gentoo/blob/master/net-im/telegram-desktop/telegram-desktop-7.3.5.ebuild

--- a/cmake/external/tdesktop_rust/CMakeLists.txt
+++ b/cmake/external/tdesktop_rust/CMakeLists.txt
@@ -5 +5,4 @@
-# https://github.com/desktop-app/legal/blob/master/LEGAL
+# https://github.com/desktop-app/legal/blob/master/LEGAL
+
+# Use the separately packaged Rust libraries instead of this aggregate.
+return()