do not setup node/yarn when not needed

[tsnobip]

This avoids

Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.

in the post job cleanup
cf https://github.com/rescript-lang/rescript/actions/runs/21551148295/job/62201302974

[pkg-pr-new]

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commit: 3c88d34