Have the integration cargo test jobs drop --target and reuse the cargo-parity cache #1395

Description

Follow-up deferred from #1259 (per-job Rust CI caches, #1258).

The integration tests and integration tests (Fastly EC lifecycle) jobs run:

cargo test --manifest-path crates/trusted-server-integration-tests/Cargo.toml --target x86_64-unknown-linux-gnu

That compiles about 376 crates, none of which are in the cargo-axum cache those jobs restore. test-parity compiles the same crates, but without --target, so they land in target/debug instead of the target-specific directory. The explicit --target dates from #442, when .cargo/config.toml set a global wasm32 build target; #643 removed that setting.

Dropping --target from those two steps and having them restore cargo-parity-${{ runner.os }} would let them reuse test-parity's build. The shared setup-rust action already takes a cache-key input, so the integration action needs a cache-key input and a small change.

Done when

• The two integration cargo test steps run without --target.
• Those jobs restore the cargo-parity cache (read-only), while prepare integration artifacts keeps cargo-axum.
• CI logs show the integration jobs reusing the parity cache rather than rebuilding the integration-tests crate's dependencies.

Affected area

CI / Tooling

Context

aram356's review on #1259 (inline seedling on .github/actions/setup-integration-test-env/action.yml, and the "File the deferred integration follow-up" note in the review body).