JoltPhysics/RapierPhysics: Fix some warning messages

[puxiao]

If you use webpack/rspack, you will receive a warning message like this:

Module parse warning:
  ╰─▶   ⚠ Critical dependency: the request of a dependency is an expression
          ╭─[41:30]
       39 │     if (RAPIER === null) {
       40 │
       41 │         RAPIER = await import(RAPIER_PATH);
          ·                               ────────────
       42 │         await RAPIER.init();
       43 │
          ╰────

Critical dependency: the request of a dependency is an expression

Therefore we cannot use a constant directly.

This warning can be resolved by using template strings.