Add buildbot's Bash script's specific environment variables to local builds

[ehmicky]

This issue is part of the bigger issue at #40. Also related to #667.

Some environment variables are used mainly inside the buildbot's internal logic. Most (but not all) are undocumented. Some are meant to be overridden by users, others not.

The list of variables is mostly private so cannot be listed in this issue. Those are mostly variables related to specific languages (Go, Node, PHP, etc.) or frameworks (Hugo, etc.).

The problem is that those variables are not present in local builds (Netlify CLI and programmatically) which might lead to local builds behaving differently from production builds.

I think the way forward is to move the Bash script's logic to @netlify/build which is a long-running item summarized here. Once this is done, the environment variables should be available both locally and in production.