ci(assign-issue): grant issues write permission to GITHUB_TOKEN

[LinXunFeng]

Description

The Issue assignment workflow fails on every newly opened issue:

Run pozil/auto-assign-issue@v1
Error: Resource not accessible by integration

The default workflow permission of the repository is read-only:

{"default_workflow_permissions": "read", "can_approve_pull_request_reviews": true}

.github/workflows/assign-issue.yml declares no permissions block, so
GITHUB_TOKEN is granted read-only access and the action cannot assign the
issue. deploy.yml was already given an explicit permission in 562cad4, this
workflow was missed.

Grant the job the single permission it needs:

jobs:
  auto-assign:
    runs-on: ubuntu-latest
    permissions:
      issues: write

The workflow only triggers on issues, so issues: write is enough,
pull-requests: write is not needed.