wiggle-wasmtime: witx paths should be relative to CARGO_MANIFEST_DIR, not WASI_ROOT

the WASI_ROOT was leftover from when this code was part of `wig`.

crates/wiggle/wasmtime/macro/src/lib.rs

@@ -48,9 +48,9 @@ use config::{MissingMemoryConf, ModuleConf, TargetConf};
 #[proc_macro]
 pub fn wasmtime_integration(args: TokenStream) -> TokenStream {
     let mut config = parse_macro_input!(args as config::Config);
-    config
-        .witx
-        .make_paths_relative_to(std::env::var("WASI_ROOT").expect("WASI_ROOT env var"));
+    config.witx.make_paths_relative_to(
+        std::env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR env var"),
+    );
     let doc = config.load_document();
     let names = Names::new(&config.ctx.name, quote!(wasmtime_wiggle));