`doctl serverless install` should not require authentication

**Describe the Issue:**

When running `doctl serverless install` without `DIGITALOCEAN_ACCESS_TOKEN` set, the command will refuse to install the `serverless` extension.

**Environment:**

- doctl version:
- OS: Pop_OS
- Installation method: Homebrew

**Additional Details:**

I run `doctl serverless install` in a Docker container when the container starts. I'd prefer moving the `serverless` extension installation directly into the container build step so that the container can start with the extension already installed. I find it poor praxis to provide the DO access token during container build time, just to have the extension installed.