Fix incorrect ETag cache key when relative instance URL is used

What does this MR do?

Fixes incorrect cache key for ETag caching when instance uses a relative URL. In this case we have to take into account both PATH_INFO and SCRIPT_NAME.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #32995 (closed)