fix overrides dropped when --multirun flag appears between them

[Mr-Neutr0n]

When --multirun (or any optional flag) is placed between override arguments on the command line, argparse only sees the overrides before the flag and rejects the rest as unrecognized. For example:

python app.py task=1 --multirun db=mysql
# error: unrecognized arguments: db=mysql

This happens because nargs="*" positional args stop consuming tokens as soon as argparse encounters an optional flag. The overrides after the flag never get picked up.

Fixed by switching from parse_args() to parse_known_args() and merging any remaining tokens back into the overrides list. All orderings now work:

python app.py --multirun task=1 db=mysql    # works (already worked)
python app.py task=1 db=mysql --multirun    # works (already worked)
python app.py task=1 --multirun db=mysql    # works (was broken)

Fixes #3053