never use /var/folders for TMPDIR on darwin

This doesn't just cause problems for nix-store --serve but also results
in certain build failures. Builds that use unix domain sockets in their
tests often fail because the /var/folders prefix already consumes more
than half of the maximum length of socket paths.

    struct sockaddr_un {
       sa_family_t sun_family;               /* AF_UNIX */
       char        sun_path[108];            /* Pathname */
    };

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