use XDG_CACHE_HOME on Linux

[apainintheneck]

• 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?

---

Closes #16156

This variable was getting filtered out before so it wasn't possible for us to use it as the default for HOMEBREW_DEFAULT_CACHE and HOMEBREW_DEFAULT_LOGS. I'm no bash expert so if there's a better way to handle this let me know.

[Bo98]

You can just add it to USED_BY_HOMEBREW_VARS I think

[MikeMcQuaid]

Makes sense, thanks @apainintheneck!