[.github] Add retries around "npm ci"

Command `npm ci` fails often enough, it's probably worth wrapping it in a `retry(3)` loop.

Example:

```
$ npm ci
...
npm error code ECONNRESET
npm error network aborted
npm error network This is a problem related to network connectivity.
npm error network In most cases you are behind a proxy or have bad network settings.
```

https://github.com/Azure/azure-rest-api-specs/actions/runs/16999992756/job/48199650548?pr=36712