Bump default host Rust toolchain to 1.89

score_baselibs and lifecycle use library APIs stabilized after the
rules_rust default host toolchain (Rust 1.86.0): is_multiple_of (1.87)
and NonNull::from_ref / str::from_utf8* inherent constructors /
array-length inference (1.89). The new lifecycle rust_supervised_app
example pulls the baselibs Rust logging stack into the x86_64 host build,
which failed to compile with 1.86.0. Ferrocene (only used for
safety-target configs) is not involved in this build.

Register Rust 1.89.0 as the default host toolchain.

Author: antonkri

bazel_common/score_rust_toolchains.MODULE.bazel

@@ -49,3 +49,17 @@ use_repo(
     ferrocene,
     "ferrocene_aarch64_ebclfsa",
 )
+
+# Pin the default host Rust toolchain above the rules_rust default (1.86.0).
+# score_baselibs / lifecycle use library APIs stabilized after 1.86
+# (is_multiple_of -> 1.87; NonNull::from_ref, str::from_utf8* inherent
+# constructors, array-length inference -> 1.89), so a newer stable toolchain
+# is required for the x86_64 host build.
+rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
+rust.toolchain(
+    edition = "2021",
+    versions = ["1.89.0"],
+)
+use_repo(rust, "rust_toolchains")
+
+register_toolchains("@rust_toolchains//:all")