Bin_client: use -- prefix for command line arguments

This simple MR aims to use -- instead of - when calling "long cmd line arguments" in the bin_client/octez-init-sandboxed-client.sh script.

Indeed, even if there is no standard (AFAIK), the - are usually used for short args, such as -f, and -- are for long args, such as --foo . Let's avoid using -foo and be misleading.

(We may want to generalize this to other scripts)

Manually testing the MR

Try the sandbox scripts

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR