Add retry to Secure danger-review jobs

Summary

There's currently a bug where the recently added danger-review job will fail to run for devopssecure repos. This is ironically because our pipelines are too performant and the job will finish before the MR is opened, see workflow reproduction below:

Steps to reproduce

1. Push up code to new branch, triggering pipeline
2. Wait several minutes until danger-review job has ran
3. Open MR - note no danger review

Example Project

See discussion gitlab-org/security-products/danger-bot!8 (comment 427874963)

What is the current bug behavior?

Danger doesn't run

What is the expected correct behavior?

Danger runs when MRs are opened

Output of checks

This bug happens on GitLab.com

Possible fixes

1. Option A: Add retry with delay to danger-review ensuring it retries if no MR is found
2. Option B: Switch to pipelines for merge requests to ensure job runs when merge request event occurs