ci (docs): fix docs lint action on forks (#35277)

Before:

The docs lint action doesn't run properly on forks because it tries to
use the branch name to identify the PR for updating, but the branch name is adapted on forks, so it is not `branch-name`, but `forking-account:branch-name`.

After:

Use the PR number instead to identify the PR instead.

.github/workflows/docs-lint-v2.yml

@@ -63,7 +63,7 @@ jobs:
         env:
           BASE_REF: ${{ github.base_ref }}
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          BRANCH_NAME: ${{ github.head_ref }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
         run: |
           set -o pipefail
           run_lints() {
@@ -76,6 +76,6 @@ jobs:
           LINT_EXIT_CODE=$?
           set -e
           if [[ $LINT_EXIT_CODE -ne 0 ]]; then
-            gh pr comment "$BRANCH_NAME" --body "$LINT_RESULTS"
+            gh pr comment "$PR_NUMBER" --body "$LINT_RESULTS"
             exit 1
           fi