Add system_sysroot to rust_toolchain for pre-installed toolchains

[rejuvenile]

Summary

• Adds a system_sysroot attribute to rust_toolchain that specifies an absolute path to a pre-installed Rust toolchain on build workers
• When set, Rust toolchain files (~670MB) are excluded from action inputs, significantly reducing transfer overhead for remote execution
• RUSTC, RUSTDOC, CARGO paths in build scripts and make variables are redirected to the system-installed binaries
• CC toolchain files and custom linker inputs are preserved as action inputs

Motivation

When using remote execution (or local builds with --disk_cache), every Rustc action packages the entire Rust sysroot as inputs. For a typical nightly toolchain this is ~670MB of files that must be uploaded, hashed, and transferred per action. If the remote workers already have the toolchain installed (e.g. via container image or system package), this transfer is pure overhead.

Setting system_sysroot = "/path/to/toolchain" on rust_toolchain tells rules_rust to use the pre-installed toolchain directly, eliminating the transfer cost.

Test plan

• Verify builds succeed with system_sysroot set to a valid rustup toolchain path
• Verify builds still succeed without system_sysroot set (default behavior unchanged)
• Verify remote execution works with pre-installed toolchain on workers

🤖 Generated with Claude Code

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[UebelAndre]

Thanks! Could this be a repository rule instead? Much like @rules_cc//cc/private/toolchain/cc_configure.bzl%cc_configure?

[rejuvenile]

@UebelAndre I screwed up this PR slightly, so recreated it at #3928. I'll make those changes there. Thanks for the quick response!