Sync fork with GitHub App token

[wata727]

See https://github.com/terraform-linters/magic-modules/actions/runs/13745172118/job/38439197950
See cli/cli#7574

The auto-sync introduced in #7 failed with the following error:

HTTP 404: Not Found (https://api.github.com/repos/GoogleCloudPlatform/magic-modules/git/refs/heads/tflint_provider)

The cause of this confusing error is that the GITHUB_TOKEN issued by GitHub Actions does not have the permissions to modify workflow files. See cli/cli#7574

This is a security feature, and to work around it you need to issue a temporary token using something like actions/create-github-app-token.

In this PR, we will use the tflint-automation app created in this organization to issue a token and use it for gh repo sync.

[wata727]

@bendrucker To support this requirement, I created a GitHub App for our organization.
https://github.com/apps/tflint-automation

The APP_ID and APP_PRIVATE_KEY were created as organization variables and secrets and can be found here.
https://github.com/organizations/terraform-linters/settings/secrets/actions
As you can see, the scope of repositories that can use APP_PRIVATE_KEY is limited, so add more if necessary.

You can check the settings installed in the organization from the link below.
https://github.com/organizations/terraform-linters/settings/installations
The repositories that can be referenced by the app are similarly restricted, so please add them if necessary.