tools: add GitHub token permissions to label flaky-test issues

Signed-off-by: Gabriela Gutierrez <gabigutierrez@google.com>
PR-URL: #45308
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: gabibguti

.github/workflows/label-flaky-test-issue.yml

@@ -4,6 +4,9 @@ on:
   issues:
     types: [opened, labeled]
 
+permissions:
+  contents: read
+
 jobs:
   label:
     if: github.event.label.name == 'flaky-test'