Test import workflow breaks when called from a fork

[AustinAbro321]

Description

We can see an example of the test import workflow breaking on a fork here - https://github.com/zarf-dev/zarf/actions/runs/10581228147/job/29404470354?pr=2933. This happens because the workflow currently tries to pull the current commit of the PR from the Zarf repo. If the commit is on a fork, then it will not exist on the Zarf repo. Instead we should check if we can import the codebase from a local directory.

Severity/Priority

High - we don't want to block outside contributors or have them think their contributions are breaking in some way.