fix: use git submodule update --init --checkout

This makes the checkout of the recorded moddable commit idempotent.

packages/xsnap/src/build.js

@@ -37,7 +37,7 @@ function exec(command, cwd, args = []) {
     }
     await exec('git', 'moddable', ['checkout', moddableCommitHash]);
   } else {
-    await exec('git', '.', ['submodule', 'update']);
+    await exec('git', '.', ['submodule', 'update', '--init', '--checkout']);
   }
 
   const pjson = readFileSync(`${__dirname}/../package.json`, 'utf-8');