Embed rtc_rust check into rtc_rust_test_suite template

for consistency with can_build_rust_unit_tests checks and to
simplify usage of such template that would always be present but be noop
when rust is disabled.

Bug: webrtc:416446214
Change-Id: I6d47ab1be56a5735f6296b638bcc724201134cf2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/501960
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48570}