If you hold your breath to the point you can't anymore your body will automatically clear your nostrils in an attempt to help you not suffocate to death
There's a trick I learned for clearing a stuffy nose.
Place the tip of your tongue on the roof of your mouth. Then, place a finger on the bridge of your nose.
In very small motions, alternate between pushing up on the roof of your mouth, and then releasing that and pushing on the bridge of the nose. Use a gentle teeter-totter to rock the pallette. Repeat ten or so times and, within a few seconds, your nose will be open.
My nostril generally gets blocked on the side closest to the person who's spreading all the germs.
But I find you can sort it out if you tilt your head back slightly and point your nose up, then tilt to the side away from the blocked nostril. Once the blood starts to even out you can level out your head with both nostrils fairly clear.