Fixes for 0.15 (#1449)

* Wasmtime 0.15.0 and Cranelift 0.62.0. (#1398)

* Bump more ad-hoc versions.

* Add build.rs to wasi-common's Cargo.toml.

* Update the env var name in more places.

* Remove a redundant echo.

crates/wasi/build.rs

@@ -1,4 +1,4 @@
 fn main() {
-    let wasi_root = std::env::var("DEP_WASI_COMMON_14_WASI").unwrap();
+    let wasi_root = std::env::var("DEP_WASI_COMMON_15_WASI").unwrap();
     println!("cargo:rustc-env=WASI_ROOT={}", wasi_root);
 }