package/scripts/postinstall: fix permissions of opt

[ZhongRuoyu]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

Fixes https://github.com/orgs/Homebrew/discussions/5052.

[MikeMcQuaid]

Thanks @ZhongRuoyu!