ci,scripts: bootstrap the Rust toolchain in check-system-test-trigger

[avi-starkware]

Problem

check-system-test-trigger in hybrid_system_test.yaml runs scripts/check_test_trigger.py, which calls cargo tree --depth 0 via scripts/tests_utils.py::get_workspace_tree(). The job installs Python but never a Rust toolchain, so cargo resolves to whatever rustup state the runner image happens to carry.

On PR #15026 that state was broken, deterministically. Three attempts of run 34341989783 on three different runners (nsc-runner-ksra8te9v8m3i, -u4nb253qddbkm, -92iapg1b1s7bo) all failed the same way:

info: syncing channel updates for 1.96-x86_64-unknown-linux-gnu
info: downloading component rust-std
error: 'cargo' is not installed for the toolchain '1.96-x86_64-unknown-linux-gnu'.

A passing run of the same job on another branch (34336680140) shows syncing channel updates and then ~50 s of work; the failing runs spend ~7 s and only fetch rust-std. rustup is finding a 1.96 toolchain it considers already installed and topping up the target, but the install has no cargo component.

Fix

Use ./.github/actions/bootstrap in the job, the same way main.yml, merge_queue_ci.yml and main_nightly.yml already do for every other job that runs these scripts. That installs the toolchain pinned by rust-toolchain.toml explicitly instead of relying on ambient runner state.

Bootstrap's namespace_cache step already mounts the python cache volume, so the job's own nscloud-cache-action step is dropped rather than mounting it twice.

tests_utils.py carried two RUSTC_WRAPPER="" overrides with TODO(Yoav): Remove this once hybrid_system_test uses bootstrap. This PR is that condition, so both overrides are removed. Bootstrap installs sccache (via install_cargo_tools.sh) and .cargo/config.toml already sets rustc-wrapper = "sccache" for every other cargo invocation, so no new requirement is introduced.

Verification

• python -c 'import yaml; ...' parses the workflow; the job's steps are now: checkout → bootstrap → setup-python → pip install → check.
• black -l 100 -t py37 --check and isort -c (the py_code_style.py flags) pass on scripts/tests_utils.py.
• Locally, get_workspace_tree() and get_package_dependencies() run without the override (115 workspace packages resolved).
• This PR edits .github/workflows/hybrid_system_test.yaml, which is in the job's path_triggers, so check-system-test-trigger runs for real here (no optimizer skip) and should return should_run=true, exercising the full path.

🤖 Generated with Claude Code

[reviewable-StarkWare]

This change is 

[avi-starkware]

• ci,scripts: bootstrap the Rust toolchain in check-system-test-trigger #15105 👈 (View in Graphite)
• main-v0.14.4

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