makeCLIResolver should use environment variables for database client secrets, not a config file

[mbostock]

Currently makeCLIResolver looks for a ~/.observablehq JSON configuration file:

https://github.com/observablehq/cli/blob/6dc67f0f5694beea934eb8256f92aa7025e3e2eb/src/resolver.ts#L37-L48

To support building pages with DatabaseClient in GitHub Actions, we’ll want to use environment variables (and repository secrets) instead. I don’t think it’s worth it supporting both — we should remove the code that reads the config file in favor of using process.env.

[mbostock]

Possibly the same as #81, although this issue is more opinionated in the recommended solution, and does not discuss secrets (which I expect will be a platform-only feature since there is no way to make them secure without platform support).