ci: restrict the job permission for security

[suzuki-shunsuke]

I forgot to set the permission in #82 .
This workflow doesn't require any permissions.
The permission should be restricted to least privileges.

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#defining-access-for-the-github_token-scopes-1

You can use the following syntax to disable permissions for all of the available scopes:

permissions: {}