PC-294: fix kondukto scan

[aliwoodman]

Ticket: https://ourfuturehealth.atlassian.net/browse/PC-294

This PR introduces Snyk scanning to the design-system-toolkit repository, replacing semgrep as recommended by @DionKontopodias

Other repos use the reusable action from the kondukto-scan repository for snyk scanning. But since the reusable action is in a private repository, we cannot directly use it in this repo which is public.

I don't have a very clear view on exactly what we are aiming for - this ticket was created because we saw the kondukto scan fail e.g here. In a thread and in some messages Dion seemed to suggest an approach like this which is what I've attempted to add

I've copied over the snyk-specific logic with a couple of changes:

• Snyksast Results: The snyksast scan is not producing any results, which was causing an error because it does not create an output file (seen in logs on this run when running the snyk commands in debug mode - e.g. '[json-file-output] The input is empty, skipping rendering!'). Instead of failing when no file is created, I'm logging this, as the command has actually succeeded at this point. The scan appears to process 35 files, which is close to the number of .js files in the repository - the project primarily consists of .njk templates, which may explain the lack of results (but I'm not totally sure).
• Unlike the original, this script fails if snyk auth fails - the original script would log but continue and the job would succeed without running scans even if scans were requested. (This may be a wider change worth making in the reusable action? Other pathways can fail too in this script and similarly not cause it to fail - will raise with Security to see if intended)

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