[proposal] Allow the hook to skip copy externals step if those have been provided by runner

[anlesk]

Prompted by:
#82 (comment)

Some of the storage systems are slow when it comes to copy files, especially when the number of files is high.
As the part of every prepare job, the hook copies the content of externals into the runner. So far, the contents of externals are node16 and node20 folders, which might take extra 3-15 minutes depending on the storage type to copy.

Would that be possible to pass a flag to skip the step and have those externals being provided by the custom runner's image?

[alecrajeev]

Yeah having this option would be super helpful for us as well. There is ~3 minute delay for us here.