This solution was introduced in !7901, but it still open without any review. More info in the original MR. Also I should note that GTK uses XDG_DATA_DIRS and XDG_HOME_DIRS envitronment variables to locate icon themes and MIME information.
This solution fixes: https://bugs.winehq.org/show_bug.cgi?id=49987 and https://bugs.winehq.org/show_bug.cgi?id=58023
-- v2: ntdll: Treat XDG_DATA_DIRS as special env variable