Use `setup-node` GitHub Actions step before running `npm publish`.

Created by: olafurpg

The last npm publish step failed with an authorization error https://github.com/sourcegraph/src-cli/runs/6707002077?check_suite_focus=true

npm ERR! need auth This command requires you to be logged in to https://registry.npmjs.org/
29

Our current hypothesis is that this change fixes the issue.

Test plan