[lit] Create one output file when `--output` is specified more than once

The argparse 'append' action concatenates multiple occurrences of an
argument (even when we specify `nargs=1` or `nargs='?'`).  This means
that we create multiple identical output files if the `--output`
argument is given more than once.  This isn't useful and we instead want
this to behave like a standard optional argument: last occurrence wins.