snapcraft pack --use-lxd ask for the optional directory

[Guillaume Beuzeboc]

The snapcraft pack command is supposed to take optional options and an optional directory.
After running `snapcraft prime --use-lxd`, I wanted to run `snapcraft pack --use-lxd`

When specifying a flag (--use-lxd in this case), the snapcraft command complains about the missing directory:

    Usage: snapcraft pack [OPTIONS] DIRECTORY
    Try 'snapcraft pack -h' for help.

    Error: No such option: --use-lxd

While `snapcraft pack --help` shows:

    Usage:
        snapcraft pack [options] <directory>
    Summary:
        Process parts and create a snap file containing the project payload
        with the provided metadata. If a directory is specified, pack its
        contents instead.
    Options:
                       ...
                       --use-lxd: Use LXD to build

I ran my test with a simple core20 snap.

snapd: 2.56.2
snapcraft: 7.1.0