Fix slack notification in api repo (maybe others)

[sastels]

Describe the bug

I think the curl used by GitHub actions changed. When the api CI tests fail we don't get the Slack notification anymore, and in the logs we see

{"detail":[{"type":"json_invalid","loc":["body",1],"msg":"JSON decode error","input":{},"ctx":{"error":"Expecting property name enclosed in double quotes"}}]}

(SEV-1 Critical, SEV-2 Major, SEV-3 Minor, SEV-4 Low)

To Reproduce

Steps to reproduce the behavior:

1. Make the Slack webhook fire
2. No notification in Slack

Expected behavior

We get a notification

Impact

We don't know when deploys fail. This could also affect manifest repo where releases could fail without us knowing.

QA

Make the webhook fire (say just in a PR)