The short answer is that soldiers would use them right-handed. The lock and the barrel were the most expensive parts of a musket, and trying to build left-handed ones and allocate them would have been impossible. This would have been another task, like writing with a quill pen, that would have been more difficult for left handed people. It likely was yet another reason why left handed children were sometimes punished to try to make them right handed: but on this, and how the military would generally react to left-handed recruits, I am not an expert.

Practically speaking, it's possible to fire a well-designed flintlock or percussion gun with the lock on the same side of the stock as your head. Double barrelled guns with two locks , especially shotguns, were not unknown. A small lip on the back of the pan helps direct the flash up and to the side, for a flintlock. For a percussion lock, being directly behind the hammer is perhaps an advantage, as any gas leakage around the hammer and nipple gets spread to the sides and the front - though, if the nipple is loose, sitting in threads that are worn, it would be better to be off to the side of its possible flight path.