Can't see anything from that pic 'cuz it's too fuzzy. Does the crack run radially or is it along the rim? If it runs along the rim I'd say the rim is toast. Does it brake smoothly or is the sidewall deformed enough that the brake pulses?

If the crack is purely radial, consider this. Rims are (or at least were) made by pulling the ends together but not necessary welding them, nor even pinning them to maintain alignment. There is LOTS of spoke tension holding the rim together. Jobst Brandt tells of an experiment where they cut a rim up into something like eight sections, then strung up a wheel with only the spoke tension holding it together. It worked just fine.

So if it trims up okay and if it brakes okay, you can probably use it in non-demanding situations. Check it OFTEN for the crack spreading. Check the rim straightness often. Don't trust your life on it descending the Pyrenees. If it grows, especially up into the spoke nipple surface, decommission it. At least you will have bought time while looking for a replacement.