Use the result of scripts.find() instead of using operator[] when
looking up scripts. This avoids an ugly const_cast, and avoids doing the
lookup twice.

Change-Id: I7c1a6be28928e2e3d928c389328be8785be3cff7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3056989
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75974}