fix: add skip_per_crate_rustc_flags to rust_crate.bzl

[rejuvenile]

Summary

Set skip_per_crate_rustc_flags = True on generated third-party crate targets in rust_crate.bzl, enabling configuration trimming for better cache hit rates.

Problem

When experimental_per_crate_rustc_flag is set in rules_rust (e.g., to pass extra flags to first-party crates matching a // prefix), all third-party crates are placed into a different Bazel configuration even though the per-crate flags never match them. This causes unnecessary rebuilds of every third-party crate whenever the flag value changes.

Fix

Pass skip_per_crate_rustc_flags = True to the rust_library / rust_proc_macro call in rust_crate(). This leverages the corresponding rules_rust attribute (added in [bazelbuild/rules_rust#XXXX]) to trim the experimental_per_crate_rustc_flag setting from the configuration of third-party crates, returning them to a canonical configuration.

Dependency

This change requires rules_rust to have the skip_per_crate_rustc_flags attribute defined. Without it, Bazel will error on the unknown attribute. The rules_rust PR adding this attribute is being submitted in parallel.

Changes

• rs/rust_crate.bzl: Add skip_per_crate_rustc_flags = True to the kwargs dict passed to rust_library/rust_proc_macro