feat: use SNYK_CFG for IaC OCI registry env vars [CFG-1165]

[teodora-sandu]

What does this PR do?

This PR changes the IaC OCI registry env vars to be set via snyk config set by using the SNYK_CFG_ prefix.

Where should the reviewer start?

The code change is pretty small and straightforward, but there will be other places we'll need to change these.
For now, the tests work without changing the CircleCI environment variables because we read them and then pass them with SNYK_CFG_ prefixed to them in the acceptance tests. I will need to update them in the public docs, 1Password, and the contract tests in https://github.com/snyk/snyk-iac-rules

How should this be manually tested?

1. Run npm run build
2. Run the CLI with no configs: snyk-dev iac test test/fixtures/iac/terraform/sg_open_ssh.tf
3. Use the details in the IaC Tests - OCI Registries Credentials vault to set the following:

snyk-dev config set oci-registry-url=
snyk-dev config set oci-registry-username=
snyk-dev config set oci-registry-password=

4. Run the CLI with the configs: snyk-dev iac test test/fixtures/iac/terraform/sg_open_ssh.tf

Any background context you want to provide?

We agreed on this in this Slack thread.

What are the relevant tickets?

https://snyksec.atlassian.net/browse/CFG-1165

Screenshots

Step 2:

Step 4:

Help docs:

[teodora-sandu]

@ipapast We can get away without changing the env vars in CircleCI (and the same for snyk-iac-rules), but I'm wondering if you think it's confusing?

[ipapast]

I think let's keep them consistent, it's not a big deal to change them in CircleCI. (we also might not remember this in a few months time :) )
+++ update in 1password

[teodora-sandu]

I've updated the variables in the code but will change them in CircleCI when I'm close to merging, so we don't break existing pipelines

[ipapast]

we should go with expand and contract; have both the new and old values on the first merge, remove after the merge. just in case someone's releasing at the same time

[teodora-sandu]

Actually, I just realised that the env vars in CircleCI have nothing to do with the SNYK_CFG env vars. They are strictly configured for the tests, so we can then pass them to the SNYK_CFG env vars. So I will keep them as they are

[ipapast]

AH!! yes, you are very right, I explicitly named them with the registry name inside (docker, etc) for this reason, I should have remembered it 😆

[ipapast]

Tested this with set, unset, clear and it works great.
We had a chat on Slack with Teo about generating docs to show the usage of these env vars too.
I'm approving as it works great, some docs will be added soon