Setting Environment Variables at Deployment

I want to deploy a project with a Node module on a private registry added as a dependency.

I configured `.npmrc` following:

```
//npm.pkg.github.com/:_authToken=${NPM_TOKEN}
@scope:registry=https://npm.pkg.github.com
```

However, I cannot pass environment variables to the build process with the `firebase deploy` command.

So the authentication error has occurred in the deployment process.

The official document said, "You can set the `$NPM_TOKEN` environment variable with the `--set-build-env-vars` argument to your `gcloud functions deploy` command."
https://cloud.google.com/functions/docs/writing/specifying-dependencies-nodejs#private_modules_from_other_repositories

However, since I am using Firebase, I need to use the `firebase` command, not the `gcloud` command.

I want to pass environment variables to the build process in the `firebase deploy` command as well.