Switch Jenkins Security Scan to cron

[offa]

There are some issues on Dependabot created PRs:

Error: Workflows triggered by Dependabot on the "push" event run with read-only access. Uploading Code Scanning results requires write access. To use Code Scanning with Dependabot, please ensure you are using the "pull_request" event for this workflow and avoid triggering on the "push" event for Dependabot branches. See https://docs.github.com/en/code-security/secure-coding/configuring-code-scanning#scanning-on-push for more information on how to configure these events.

Builds fail on main, so excluding Dependabot push events wasn't enough.