It's notifications could be configured on "build" level, not "build job" level, i.e. it's not possible to trigger notifications for specific jobs, only entire build.
However, I see a few solutions/workarounds here. First, if you don't "care" about the results of other jobs, you can declare them under matrix.allow_failures, so only the job you are interested in will determine the result of the entire build and trigger "right" notification.
Another option would be programmatically send notification in on_failure section of that job. If it's Slack it could be done in curl or PowerShell one-liner I believe.