Make @netlify/config return site environment variables in local builds

[ehmicky]

Site environment variables come from different sources: the site, the team, and addons.

Those environment variables are used by several parts of the build: the build command and plugins, but those are also used as the default value of @netlify/build options and @netlify/config options. Those are also used by CLI commands.

At the moment, @netlify/config does not compute/merge/return environment variables. @netlify/build does take build.environment from netlify.toml into account, but not site/team/addons ones.

@netlify/config should compute/merge environment variables from the site, team and addons, then return it as an env object. This should not happen if mode is buildbot. This env object should then be used by:

• the Netlify CLI, replacing the current logic
• @netlify/build. We should make sure it is currently merged in the current logic. For example, it should not be possible to override readonly environment variables like CONTEXT (see existing logic).

In order to compute those, some additional API calls will need to be performed by @netlify/config, in addition to getSite which is already currently performed in local builds.

@erezrokah

[ehmicky]

Done at #2040.