[accelerator] fix "Go back" button breaking payment flow

frontend/src/app/components/accelerate-checkout/accelerate-checkout.component.ts

@@ -202,6 +202,7 @@ export class AccelerateCheckout implements OnInit, OnDestroy {
   }
 
   moveToStep(step: CheckoutStep): void {
+    this.processing = false;
     this._step = step;
     if (this.timeoutTimer) {
       clearTimeout(this.timeoutTimer);