doctl serverless install should not require authentication

[larsblumberg]

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.