Add Snyk scanning & monitoring

[ryan-wren]

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Internal Checklist

• I am requesting a review from my own team as well as the owning team
• I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

=======

• Added vulnerability-scan job to the CircleCI config, to run Snyk scans

Rationale

=========

The Snyk scans should only create a snapshot during CI when the changes are on the main branch.

Considerations

==============

Why you made some of the technical decisions that you made, especially if the
reasoning is not immediately obvious

Screenshots

============

Before

Image or [gif](https://giphy.com/apps/giphycapture)

After

Image or gif where change can be clearly seen

Here are some helpful tips you can follow when submitting a pull request:

1. Fork the repository and create your branch from main.
2. Run make build in the repository root.
3. If you've fixed a bug or added code that should be tested, add tests!
4. Ensure the test suite passes (make test).
5. The --debug flag is often helpful for debugging HTTP client requests and responses.
6. Format your code with gofmt.
7. Make sure your code lints (make lint). Note: This requires Docker to run inside a local job.

[vallieres]

Some changes but you are on the right track 😄

[vallieres]

For testing purposes, I would switch the condition around the branch, to run run a snyk monitor, and then go here:
https://app.snyk.io/org/circleci-public/projects?groupBy=targets&searchQuery=circleci-cli&sortBy=highest+severity&filters%5BShow%5D=&filters%5BIntegrations%5D=&before&after
and delete the newly created projects.

This way you can test it without merging to main.

[ryan-wren]

Messed up during git rebase 🤦🏾‍♂️ . I'll open a new PR with these changes.

[ryan-wren]

Closing this PR in favor of #978