build: Use -Oz for Rust toolchains with prebuilt targets

This bring those in line with the non-prebuilt target toolchains.

Change-Id: I6e318095f9840e0b6a6878d94ac546f0e1c42388
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/263096
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Pigweed-Auto-Submit: Erik Gilling <konkers@google.com>
Docs-Not-Needed: Erik Gilling <konkers@google.com>

Author: konkers

MODULE.bazel.lock

@@ -79,6 +79,11 @@ rust_toolchain(
     dylib_ext = "{dylib_ext}",
     exec_compatible_with = {exec_compatible_with},
     exec_triple = "{exec_triple}",
+    opt_level = {{
+      "dbg": "0",
+      "fastbuild": "0",
+      "opt": "z",
+    }},
     rust_doc = "{toolchain_repo}//:bin/rustdoc",
     rust_std = "{target_repo}//:rust_std",
     rustc = "{toolchain_repo}//:bin/rustc",