Slash command "/format" doesn't work for external contributors

[seisman]

Description of the problem

#766 (comment) doesn't trigger the format command. Instead, the slash dispatch action reports:

Run peter-evans/slash-command-dispatch@v2
  with:
    token: ***
    commands: format
  
    issue-type: pull-request
    reaction-token: ***
    reactions: true
    permission: write
    allow-edits: false
    repository: GenericMappingTools/pygmt
    event-type-suffix: -command
    dispatch-type: repository
Using configuration from yaml inputs.
Command 'format' is not configured for the user's permission level 'none'.

It seems something wrong with the permission setting.