Breath on your fingers like you're trying to fog up glass. Immediately open by running your fingers in opposite directions along the edge, using the additional friction you created.
Well I hope you're going to wash them anyway. There's already a bunch more worse shit than a bit of breath condensation from a guy standing a few feet away breathing onto his hands.
If you're that concerned about every "troglodyte" out there doing anything that may remotely spread a couple of germs, I advise you wear a hazmat suit when you go out. And when you're inside. Just, always. Life's filthy, take precautions before you put something in your body (wash it, cook it, etc) but past that, man.. good luck.
A "modicum" of sanitary practices doesn't include something as innocuous as using some breath condensation to open a bag lol. Say that to the parents not watching as their kid snots all over the place. If you wanna call someone a troglodyte, maybe reserve it for the guy who sneezes without covering. The level of harm someone does by selecting their produce, tearing off a bag, and God forbid breathing on their hands is actually nil.
You can call it "innocuous" all you want, it's still an unnecessarily unsanitary thing to do. Just because there's worse examples of gross people doesn't mean getting your saliva on produce in the store isn't also gross.