I claim the ornamental design for a nugget-shaped headpiece, as shown and described.
FIG. 1 is a left side elevation view of the nugget-shaped headpiece.
FIG. 2 is a right side elevation view of the claimed design.
FIG. 3 is a rear elevation view of the claimed design.
FIG. 4 is a front elevation view of the claimed design.
FIG. 5 is a top plan view of the claimed design; and,
FIG. 6 is a bottom plan view of the claimed design. FIG. 6 includes features shown in broken lines, which are deemed as environmental matter forming no part of the claimed design.