Skip `setup-node` if Node is not needed?

[jtpio]

What happens if there is no lock file, e.g. a repo does not include frontnend code? I think jupyter-ai repo is one example that falls in that category. Should we skip installing node in that case?

Originally posted by @krassowski in #266 (comment)

For Python monorepos probably it would make sense to not install Node. But not sure how disruptive this could be for those assuming Node was always available.

Originally posted by @jtpio in #266 (comment)

[krassowski]

Maybe we could start by adding a setting allowing to skip it altogether? Then we could still improve the runtime of some of the CI jobs without risking breaking anything and then we can grep repos to see how many adopted opt-out to decide it it is worth trying to be clever about it by default.