Limit top-level permissions for labeler workflow

By default the top-level `GITHUB_TOKEN` has all permissions[^1]. This
change removes all permissions by default, as recommended by the GitHub
Actions security hardening guide[^2].

This change is a low risk test. I plan to modify all other workflows if
this works okay.

[^1]:
https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token
[^2]:
https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#restricting-permissions-for-tokens

.github/workflows/labeler.yml

@@ -3,6 +3,8 @@ name: Pull Request Labeler
 on:  # yamllint disable-line rule:truthy
   - pull_request_target
 
+permissions: {}
+
 jobs:
   triage:
     permissions: