Extend start-proxy.yml to test multiple registry support

[mario-campos]

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Workflow types:

• Advanced setup - Impacts users who have custom CodeQL workflows.
• Managed - Impacts users with dynamic workflows (Default Setup, Code Quality, ...).

Products:

• Code Scanning - The changes impact analyses when analysis-kinds: code-scanning.
• Code Quality - The changes impact analyses when analysis-kinds: code-quality.
• Other first-party - The changes impact other first-party analyses.

Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

How did/will you validate this change?

• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).
• Other - The jq command was manually tested to ensure that it behaves as expected.

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Development/testing only - This change cannot cause any failures in production.

How will you know if something goes wrong after this change is released?

• Other - If something goes wrong with the modification to the test, I expect the test to fail.

Are there any special considerations for merging or releasing this change?

• Special considerations - As this PR modifies to the test to support multiple private registries of the same kind, we should first ensure that the code supports that feature before merging this PR. Otherwise, the test may fail until support is added.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[mbg]

@mario-campos You need to re-generate the actual workflow files from the updated template by running ./sync.sh in the pr-checks folder.