fix(workflows/labeler): grant "issues: write" permission

[caugner]

Summary

The label-by-size job is failing silently, because it doesn't have the issues: write permission, which it needs, because pr-size-labeler currently uses the issues API (PATCH issues/{id}) rather than the issue label API (POST issues/{id}/labels).

Test results and supporting details

Related issues

Same PR in mdn/content:

• chore(ci): Add 'issue: write' perms to workflow content#31759

Upstream PR removing the dependency on that permission:

• fix: use the add label API rather than patching the entire issue CodelyTV/pr-size-labeler#89

[caugner]

Self-approval, as mdn/content has the same permission granted.