Toolchain auto-update: fix comment posting (#3415)

actions/github-script uses Javascript, so we need to use Javascript
rather than YAML syntax in the body of the script. Also, update to
version 7 to avoid deprecation warnings.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

.github/workflows/toolchain-upgrade.yml

@@ -50,22 +50,21 @@ jobs:
 
  - name: Add debugging hints
  if: ${{ steps.create_pr.outputs.pull-request-number }}
- uses: actions/github-script@v6
+ uses: actions/github-script@v7
  with:
  script: |
  github.rest.issues.createComment({
  issue_number: ${{ steps.create_pr.outputs.pull-request-number }},
  owner: context.repo.owner,
  repo: context.repo.repo,
- body: >
- This is an automatically generated pull request. If any of the CI checks fail,
+ body: `This is an automatically generated pull request. If any of the CI checks fail,
  manual intervention is required. In such a case, review the changes at
  https://github.com/rust-lang/rust from
  https://github.com/rust-lang/rust/commit/${{ env.current_toolchain_hash }} up to
  https://github.com/rust-lang/rust/commit/${{ env.next_toolchain_hash }}. The log
  for this commit range is:
 
- ${{ env.git_log }}
+ ${{ env.git_log }}`
  })
 
  - name: Create Issue