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Reason for revert: Missing riscv support and out of date base64 crate causing brekages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: I90a4d05d7b69c02d03407d8304cd6cdebed6f9ce
diff --git a/pvmfw/avb/src/lib.rs b/pvmfw/avb/src/lib.rs
index d83737f..8fea162 100644
--- a/pvmfw/avb/src/lib.rs
+++ b/pvmfw/avb/src/lib.rs
@@ -15,6 +15,8 @@
 //! A library implementing the payload verification for pvmfw with libavb
 
 #![cfg_attr(not(test), no_std)]
+// For usize.checked_add_signed(isize), available in Rust 1.66.0
+#![feature(mixed_integer_ops)]
 
 mod descriptor;
 mod error;