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>
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