Fix the ug+metal build against the guard-based encoder API (#3855)

UgIOp1::metal_fwd calls set_output_buffer and dispatch_threads directly
on the CommandsGuard, which only forwards pipeline and label calls since
the concurrent-dispatch rework, so --features ug,metal has not compiled
since 0.11.

Deref to the underlying ComputeCommandEncoder, matching how the other
metal call sites use the guard.

Co-authored-by: ivarflakstad <69173633+ivarflakstad@users.noreply.github.com>