ci: temporarily remove rust from Ubuntu

This is for the purpose of getting an easy-to-use base for future
development.  The plan is:
- that Debian will require trixie to enable Rust usage
- that Ubuntu will backport 1.83 to its 22.04 and 24.04 versions
  (https://bugs.launchpad.net/ubuntu/+source/rustc-1.83/+bug/2120318)

Marc-André is working on adding Rust to other CI jobs.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250908105005.2119297-2-pbonzini@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

.gitlab-ci.d/buildtest.yml

@@ -39,9 +39,9 @@ build-system-ubuntu:
     job: amd64-ubuntu2204-container
   variables:
     IMAGE: ubuntu2204
-    CONFIGURE_ARGS: --enable-docs --enable-rust
+    CONFIGURE_ARGS: --enable-docs
     TARGETS: alpha-softmmu microblazeel-softmmu mips64el-softmmu
-    MAKE_CHECK_ARGS: check-build check-doc
+    MAKE_CHECK_ARGS: check-build
 
 check-system-ubuntu:
   extends: .native_test_job_template