Don't write test report if workflow is triggered by a fork

[fujidaiti]

Related issues (optional)

None.

Description

According to dorny/test-reporter:

Workflows triggered by pull requests from forked repositories are executed with read-only token and therefore can't create check runs.

As a workaround, this PR modifies the workflow to generate a test report only when triggered by the original repository.

Summary (check all that apply)

• Modified / added code
• Modified / added tests
• Modified / added examples
• Modified / added others (pubspec.yaml, workflows, etc...)
• Updated README
• Contains breaking changes
  • Created / updated migration guide
• Incremented version number
  • Updated CHANGELOG