rust: Don't specify allocator for >= 1.20.0
rust: Don't specify allocator for >= 1.20.0

System allocator is the default in rust.

starlab-io/meta-rust,starlab-io/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust