[skip ci] Fix nightly notification workflow url

jobs_url is a link to the api, rather than the website.

Also tweak wording, as we now only send one notification per workflow, rather
than per failed job.

Author: iluuu1994

.github/workflows/nightly-results.yml

@@ -13,4 +13,4 @@ jobs:
       - run: |
           export DEBIAN_FRONTEND=noninteractive
           sudo apt-get install -y curl
-          curl -X POST -H 'Content-type: application/json' --data '{"attachments": [{"text": "Job in *nightly* failed", "footer": "<${{ github.event.workflow_run.jobs_url }}|View Run>", "color": "danger", "mrkdwn_in": ["text"]}]}' ${{ secrets.ACTION_MONITORING_SLACK }}
+          curl -X POST -H 'Content-type: application/json' --data '{"attachments": [{"text": "Job(s) in *nightly* failed", "footer": "<${{ github.event.workflow_run.html_url }}|View Run>", "color": "danger", "mrkdwn_in": ["text"]}]}' ${{ secrets.ACTION_MONITORING_SLACK }}