app/findCmd: handle singular arg

[proppy]

Noticed as part of https://github.com/The-OpenROAD-Project/OpenROAD/pull/2389 that the findCmd* helpers don't seem to discard singular argument (i.e: when the flag is the only remaining argument).

This is not strictly necessary, since this information is also convoyed using argc but might be good for consistency (discard arguments both when it's the only one left and when there are others remaining).