[libcxx][lit] Simplify parsing of trailing executor arguments

Adding a positional argparse.ONE_OR_MORE arguments will correctly remove
the "--" separator after --env and parse only the command. This also has
the advantage that misspelled flags raise an argparse error rather than
silently being added to the command to be executed.

I discovered this while adding a new commandline option to ssh.py to allow
passing additional arguments to the scp/ssh commands since this is required
for our CHERI CI where we need to pass `-F <custom_config_file>` to each
ssh/scp command to set various arguments such as the localhost port, usage
of controlmaster, etc. to speed up connections to our emulated QEMU systems.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D84096