Currently slack messages for errors fail:
https://github.com/tailscale/tailscale/actions/runs/6159104272/job/16713248204
```
Error: Unexpected token
in JSON at position 151
```
This is likely due to the line break in the text. Restructure the
message to use separate title/text and fix the slack webhook body.
Updates #cleanup
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>