bootstrap/rustbuild: `--test-args` has no effect for cargo tests

[RalfJung]

I tried running ./x.py test src/tools/cargo --test-args rustc, expecting that only tests with "rustc" in their name would be run. However, this has no effect. It looks like --test-args is just ignored.

[jyn514]

Looks like duplicate of #72536

[jyn514]

This has been fixed, x t src/tools/cargo --stage 0 --test-args util::auth::Secret only runs 3 tests for me locally.