Revert "Update source for Rust 1.66.1"

Revert submission 2421642-rust-update-prebuilts-1.66.1

Reason for revert: Missing riscv support and out of date base64 crate causing brekages

Reverted changes: /q/submissionid:2421642-rust-update-prebuilts-1.66.1

Change-Id: Id61258746330ca5cd09e04376dedca55acff2890

system/rust/src/lib.rs

@@ -15,6 +15,8 @@
 //! The core event loop for Rust modules. Here Rust modules are started in
 //! dependency order.
 
+#![feature(mixed_integer_ops)]
+
 #[cfg(feature = "via_android_bp")]
 mod do_not_use {
     // DO NOT USE