diagnostic: fix XDG_DATA_DIRS check

[gromgit]

1. Pass XDG_DATA_DIRS through so that it can be checked.
2. Promote the check to cross-platform status, as an incorrect XDG_DATA_DIRS also affects macOS formulae.

Complements #17932, addresses Homebrew/homebrew-core#179217 (comment).

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[MikeMcQuaid]

As discussed previously: I want to keep this as Linux-only. Have amended the commit to do so.

Thanks for the PR @gromgit, thanks! I'll address other comments 🔜.