Leave behind the unmodified XDG_CURRENT_DESKTOP variable

Editor assessment

The subject states the preservation: the unmodified XDG_CURRENT_DESKTOP variable is left behind. The fix type suits the environment defect, and the absent scope is tolerable. No body is present, which is acceptable because the subject names the exact variable and the corrected treatment completely, though the desktop-integration breakage from modifying it would have grounded it. The PR reference is well-formed. Imperative mood, no trailing period, slightly long subject. A precise, self-sufficient platform-fix message.