#649 test(ops): prove cargo.sh keeps a host rustup shim working

Follow-up to #601, which merged without a regression test.

ops/cargo.sh used to export the private RUSTUP_HOME before asking the cargo on PATH whether it could run. On a rustup shim that fails with "no default toolchain configured" even when the host toolchain is fine.

ops/cargo-host-shim.test.mjs puts that shim first on PATH, points RELAYFLOWS_TOOLCHAIN_HOME at an empty directory, and requires ops/cargo.sh --version to print the host shim's version. It also restores the trailing newline #601 dropped.

Local: node --test ops/cargo-host-shim.test.mjs passed.

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[!NOTE]
Low Risk
Changes are limited to a new Node regression test, CI wiring, and a newline in ops/cargo.sh; no bootstrap logic changes in this diff.

Overview
Adds a regression test for the #601 fix: when a host cargo rustup shim is first on PATH and the private toolchain home is empty, ops/cargo.sh --version must still succeed and report the host shim’s version (not fail with “no default toolchain configured”).

ops/cargo-host-shim.test.mjs builds a fake shim that errors if RUSTUP_HOME points at an empty private rustup home, then runs ops/cargo.sh with RELAYFLOWS_NO_TOOLCHAIN_INSTALL=1.

.github/workflows/ops-cargo-shim.yml runs that test on Node 22 for PRs/pushes touching ops/cargo.sh, the test file, or the workflow (checkout uses the PR head SHA).

ops/cargo.sh only restores a trailing newline after the final exec line.

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Summary by cubic

Adds a regression test for the #601 fix, proving ops/cargo.sh still works with a host rustup shim even when the private toolchain's RUSTUP_HOME is empty, and restores the trailing newline dropped in #601.

• CI Adds a workflow that runs the test on changes to ops/cargo.sh, the test, and the workflow itself, since an ops-only change would otherwise skip the existing workflows.