Fix automatic toolchain update

1. Adjust permissions that were made overly restrictive in 4b829ab,
resulting in an inability to create branches.
2. Ensure `gh` can actually access the repository information by setting
`GH_TOKEN`.

.github/workflows/toolchain-upgrade.yml

@@ -9,8 +9,10 @@ on:
  workflow_dispatch: # Allow manual dispatching for a custom branch / tag.
 
 permissions:
- contents: read
+ checks: write
+ contents: write
  issues: write
+ pull-requests: write
 
 jobs:
  create-toolchain-pr:
@@ -31,6 +33,7 @@ jobs:
  current_toolchain_epoch=$(date --date $current_toolchain_date +%s)
  next_toolchain_date=$(date --date "@$(($current_toolchain_epoch + 86400))" +%Y-%m-%d)
  echo "next_toolchain_date=$next_toolchain_date" >> $GITHUB_ENV
+ GH_TOKEN=${{ github.token }}
  if gh issue list -S \
  "Toolchain upgrade to nightly-$next_toolchain_date failed" \
  --json number,title | grep title ; then