First define an envelope with 1 second attack, and set the modulation marker (the little downwards pointing triangle) to 20 ms.

and then use the Keyboard Time Modulation curve to define how this modulation is mapped across the keyboard:

That’s it ! With these settings, attack time will be 1 second for midi note C3, and 20ms for midi note C8. In between, it will move linearly from 1 second down to 20 ms.

Similarly, the Velocity Time Modulation curve can be used to control envelope times with velocity. And remember, each segment of the envelope has his own modulation marker !