Fix back ports getting tagged as latest on NPM + react-native-macos-init publish

[Saadnajmi]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

This PR fixes two changes:

1. The react-native-macos-init publish job was missing a step to set up some tools, like node and ruby. This caused the pipeline to fail. This should be a relatively simple fix.

2. Backports to 0.68-stable were getting tagged as "latest" on NPM. This is because our Publish pipeline has a variable latestStableBranch defined in its build settings, set to 0.68-stable. I thought I could override that locally in vars.yml, but that change didn't carry down to the 068-stable branch. Therefore, when a new change was published to 0.68-stable, it still thought that was the latest released and tagged it on NPM as such. To fix this, we can remove the variable from our yml, add back the note it's expected to be defined in the pipelines' build settings, and update the build settings. I don't have pipeline edit auth, so I'm asking @dannyvv to do so 🙂.

Changelog

[INTERNAL] [FIXED] - Fix back ports getting tagged as latest on NPM

Test Plan

CI should pass.