Use correct context for accessing variables

Author: t-wright

.github/workflows/changelog.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: Automattic/vip-actions/changelog@f083b1e1e0cd092ee5cafa6621d32a28c68bf7ee # trunk
         with:
-          endpoint: ${{ env.CHANGELOG_POST_ENDPOINT }}
+          endpoint: ${{ vars.CHANGELOG_POST_ENDPOINT }}
           endpoint-token: ${{ secrets.CHANGELOG_POST_TOKEN }}
           repo-token: ${{ secrets.GITHUB_TOKEN }}
           status: 'draft'