Hammer it back in, then get a small clip to push through that tiny hole on the inside of the caliper (like the other pin you can see has). Driving as it is isn't an option as I suspect you're very close to losing the retaining clip, if that pops off you'll find the brake pads will want to move around in the caliper (although they won't fall out as the other pin is there), but pushing the pin back in should be safe enough to get you to a shop to buy the clip.
Any bit of metal you can push through will do, I've used hair clips before in an emergency.