Allow nftest to freeze the source code before running

[zhuchcn]

I also found it super annoying, that it fails when I checkout to another branch when a nextflow run is running. I'm wondering if there is anything we can implement to the nftest. Maybe we can do something like create a frozen copy of the pipeline when calling nftest run. Then even if we checkout the branch it still will be running from the frozen version of the pipeline.

Originally posted by @zhuchcn in https://github.com/uclahs-cds/group-infrastructure/discussions/5#discussioncomment-3244387

[zhuchcn]

Commented by @yashpatel6

That would make sense, we can set up a clone step into a temp directory and run the pipeline with that it's not affected by the actual branch version.