tools: add GitHub token permissions to label flaky-test issues
Add minimum GITHUB_TOKEN permissions for label-flaky-test-issue.yml
workflow.
Motivation: Setting minimum permissions on workflow's top level is good practice. Similar changes were previously discussed in https://github.com/nodejs/node/pull/43743. Since label flaky-test issues workflow was recently added, this PR is a small update to restrict it's permissions.
About me: I'm Gabriela and I work on behalf of Google and the OpenSSF suggesting supply-chain security changes :)
Signed-off-by: Gabriela Gutierrez gabigutierrez@google.com