refactor: change behavior of ci when external pr raised

[jhkimqd]

Description

This PR aims to fix the CI behavior to handle github secrets for attach-additional-cdks action when a PR is raised from a fork.

The changes will however skip the deployment of a third PP network and the following bridge scenarios because the SP1 key will not be available.

[leovct]

This is what I see from my fork where the SP1 key is not set:

https://github.com/leovct/kurtosis-cdk/actions/runs/12809027341/job/35713008062

In your fork, on this specific branch, it looks like the step is not failing anymore but I'm not sure if this is because of the fix or because the SP1 key is already set in your repo? Can you confirm it's not set?

https://github.com/jhkimqd/kurtosis-cdk/actions/runs/12905389279/job/35984470854

If it's not set than lgtm!

[jhkimqd]

Thanks for checking! I had the SP1 secret set, so I've removed my SP1 secret and reran the tests, and that particular step seems to be passing normally
https://github.com/jhkimqd/kurtosis-cdk/actions/runs/12905389279/job/36455734449

[jhkimqd]

It seems like the steps requiring sp1 key is being skipped successfully
https://github.com/0xPolygon/kurtosis-cdk/actions/runs/13066519637/job/36459720972