GPG encryption fails due to invalid flag

Running the do-backup action when configuring "gpg_public_key" to enable asymmetrical GPG encryption leads to a failure due to using the wrong flag.

The charm is using "--gpg-key" to enable gpg encryption [1], but the actual flag to do this in duplicity is "--encrypt-key" [2]

Full output of the action below, note the error: "no such option: --gpg-key"

  message: "Command \"['duplicity', '--rsync-options=-e \"ssh -i /root/.ssh/duplicity_id_rsa\"',
    'full', '/home/ubuntu/important', 'rsync://<USER>@<HOST>:22/mybackups/duplicity-1',
    '--gpg-key=<REDACTED>']\" failed with return code
    \"2\" and error output:\nUsage: \n duplicity [full|incremental] [options] source_dir
    target_url\n duplicity [restore] [options] source_url target_dir\n duplicity
    verify [options] source_url target_dir\n duplicity collection-status [options]
    target_url\n duplicity list-current-files [options] target_url\n duplicity cleanup
    [options] target_url\n duplicity remove-older-than time [options] target_url\n
    \ duplicity remove-all-but-n-full count [options] target_url\n duplicity remove-all-inc-of-but-n-full
    count [options] target_url\n\nBackends and their URL formats:\n cf+http://container_name\n
    \ file:///some_dir\n ftp://user[:password]@other.host[:port]/some_dir\n ftps://user[:password]@other.host[:port]/some_dir\n
    \ hsi://user[:password]@other.host[:port]/some_dir\n imap://user[:password]@other.host[:port]/some_dir\n
    \ rsync://user[:password]@other.host[:port]::/module/some_dir\n rsync://user[:password]@other.host[:port]/relative_path\n
    \ rsync://user[:password]@other.host[:port]//absolute_path\n s3://other.host[:port]/bucket_name[/prefix]\n
    \ s3+http://bucket_name[/prefix]\n scp://user[:password]@other.host[:port]/some_dir\n
    \ ssh://user[:password]@other.host[:port]/some_dir\n swift://container_name\n
    \ tahoe://alias/directory\n webdav://user[:password]@other.host/some_dir\n webdavs://user[:password]@other.host/some_dir\n
    \ gdocs://user[:password]@other.host/some_dir\n pydrive://<email address hidden>/some_dir\n
    \ mega://user[:password]@other.host/some_dir\n copy://user[:password]@other.host/some_dir\n
    \ dpbx:///some_dir\n onedrive://some_dir\n azure://container_name\n b2://account_id[:application_key]@bucket_name/[some_dir/]\n
    \ mf://user[:password]@other.host/some_dir\n\nCommands:\n cleanup <target_url>\n
    \ collection-status <target_url>\n full <source_dir> <target_url>\n incr <source_dir>
    <target_url>\n list-current-files <target_url>\n restore <source_url> <target_dir>\n
    \ remove-older-than <time> <target_url>\n remove-all-but-n-full <count> <target_url>\n
    \ remove-all-inc-of-but-n-full <count> <target_url>\n verify <target_url> <source_dir>\n\nduplicity:
    error: no such option: --gpg-key\n"

[1] https://git.launchpad.net/charm-duplicity/tree/src/lib/lib_duplicity.py#n127
[2] https://linux.die.net/man/1/duplicity