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/crates/cargo-c/build.rs

@@ -0,0 +1,5 @@
+// Compile C with the `cc` crate: natively for the unit test, and with the
+// WASI clang (`CC_wasm32_wasip2`) for the actor.
+fn main() {
+    cc::Build::new().file("src/add.c").compile("add");
+}