steam: restore and document timezone hack (#379633)

      export LIBVA_DRIVERS_PATH=/run/opengl-driver/lib/dri:/run/opengl-driver-32/lib/dri

      export VDPAU_DRIVER_PATH=/run/opengl-driver/lib/vdpau:/run/opengl-driver-32/lib/vdpau

      # Steam gets confused by the symlinks to bind mounts to symlinks /etc/localtime ends up being, so help it out.

      # See also: https://github.com/flathub/com.valvesoftware.Steam/blob/28481f09f33c12b6ac7421d13af9ed1523c54ec4/steam_wrapper/steam_wrapper.py#L160

      if [ -z ''${TZ+x} ]; then

        new_TZ="$(readlink -f /etc/localtime | grep -P -o '(?<=/zoneinfo/).*$')"

        if [ $? -eq 0 ]; then

          export TZ="$new_TZ"

        fi

      fi

      set -a

      ${lib.toShellVars extraEnv}

      set +a