It is not an RL circuit, it is an LC. But the normal switching frequency of a buck regulator is much higher than the LC resonant frequency (L is the inductor and C is the output filter capacitance) so the LC frequency is not a factor in determining the inductor current.
The current through the inductor di = V*dt/L. When the converter switch is on, the voltage (V) across the inductor is basically constant since the output voltage varies only slightly for each switch cycle (equal to the output ripple voltage). And a constant voltage across an inductor will produce a linear change in inductor current with time.