cargo: compile C for wasm32-wasip2 with a WASI clang

The Nix package, the dev shell, and the test check set CC_wasm32_wasip2
and AR_wasm32_wasip2 to the clang of pkgsCross.wasi32 (wasi-libc), so
build scripts with the cc crate work, for example rusqlite with bundled
SQLite. The runtime-toolchain check links C into a probe with the
packaged host and an empty PATH.

cli/tests/host.rs

@@ -769,6 +769,22 @@ async fn cargo_actor_runs_unit_tests_by_default_and_can_skip_them() {
     host.stop().await;
 }
 
+#[tokio::test]
+async fn cargo_actor_compiles_c_code_for_wasi() {
+    let world = create_world("cargo-c");
+    let (host, mut mailbox) = start(&world).await;
+    let source = cargo_source("cargo-c");
+    let id = host.broker.send("console", Some("cargo"), None, None, json!({"type": "build", "source": source.display().to_string()}));
+    let built = receive_reply(&mut mailbox, &id, Duration::from_secs(300)).await;
+    assert_eq!(built.body["type"], "built", "{built:?}");
+    assert_eq!(built.body["tests"]["passed"], 1, "{built:?}");
+    let wasm = host.world.resolve("console", built.body["wasm"].as_str().unwrap()).unwrap();
+    spawn_type(&host, &mut mailbox, "c-add", &wasm, &source).await;
+    let sum = ask_type(&host, &mut mailbox, "c-add", json!({"type": "add", "a": 2, "b": 3}), "sum").await;
+    assert_eq!(sum.body["value"], 5);
+    host.stop().await;
+}
+
 #[tokio::test]
 async fn rejects_invalid_actor_ids() {
     let world = create_world("invalid-id");