Wire rust_objcopy into the generated sysroot action inputs (#3972)

## Summary

#3727 added a `rust_objcopy` attribute to `rust_toolchain`, a separate
`rust-objcopy` filegroup, and automatic opt-in for Rust 1.84+/recent
nightlies. But the glue to turn that attribute into a real action input
is missing: `ctx.file.rust_objcopy` is stashed on `ToolchainInfo` and
never used again. It never flows through `_generate_sysroot` into
`direct_files`, so it doesn't join `toolchain.all_files` and isn't
declared as an input to the Rustc action.

On Linux/macOS with symlink-based sandboxing this is masked — rustc
happens to see the neighboring file in the unsandboxed `rules_rust`
external repo. Under remote execution, Windows (file-copy sandbox), or
stricter local sandboxes, rustc invokes `rust-objcopy` and fails:

```
error: unable to run `rust-objcopy`: No such file or directory (os error 2)
```

Tracking: #3307.

## Fix

Mirror the `linker` handling in `_generate_sysroot`: symlink
`rust_objcopy` into the sysroot at
`lib/rustlib/<triple>/bin/rust-objcopy` (where rustc looks) and append
it to `direct_files` so it becomes a declared Rustc action input.

## Verification

Patched `rules_rust` via `local_path_override` in a minimal smoke
workspace using rustc 1.93.0 on aarch64-apple-darwin:

- `bazel build -c opt //:hello` succeeds (opt-mode `process_wrapper`
uses `-Cstrip=debuginfo`, which invokes rust-objcopy).
- `bazel aquery 'mnemonic("Rustc", //:hello)'` now lists the sysroot
`rust-objcopy` symlink as a declared input — it was absent before.

Before: no `rust-objcopy` entry in the Rustc action inputs.
After:
`bazel-out/.../rust_toolchain/lib/rustlib/aarch64-apple-darwin/bin/rust-objcopy`.