chore: set registry-url to npmjs for release-please

Hey @SimenB, I tried something to fix the release, in the [documentation](https://docs.github.com/en/actions/tutorials/publish-packages/publish-nodejs-packages#publishing-packages-to-the-npm-registry) of the node-setup action, they do state that you need to specify a registry url: > Please note that you need to set the registry-url to https://registry.npmjs.org/ in setup-node to properly configure your credentials. Without it, I believe that the .npmrc file is not created, and therefore npm publish might not be using the auth token. Which also seems to correspond to what I pointed out in my [comment](https://github.com/istanbuljs/babel-plugin-istanbul/pull/301#issuecomment-3333148411). I'm not 100% sure, the release might also be failing because we are now using a different registry URL, and therefore the NPM_TOKEN in the secrets might not be the correct one for this new registry.