This has been removed recently (https://github.com/doitsujin/dxvk/issues/5330). Use mangohud instead:

MANGOHUD_CONFIG="fps_limit=120,preset=0" mangohud %command% (Preset 0 hides mangohud; use 1 to show FPS only, or any other parameters.)

OLD ENTRY:
For games that do not support frame rate limiting natively or only VSync on/off (with on feeling sluggish, and off “melting” the GPU), use DXVK_FRAME_RATE=120 %command% in Steam’s custom launch command setting.