Add `esy.sandboxEnv`

[andreypopp]

Add new key in package.json called esy.sandboxEnv.

Motivation:

• Set OCAMLRUNPARAM=b so we can have stack traces.
• Override C compiler (e.g. choose between clang or gcc)

It should have the same format as esy.exportedEnv but the mechanics should be the following:

• Only top level package's esy.sandboxEnv is considered.
• Env vars in esy.sandboxEnv should be propagated to all package's builds.
• Make sure those env vars participate in build cache key.

See https://github.com/jordwalke/esy/blob/9eed98f8024c3a64502387f021db7fe10ed0680a/lib/Sandbox.js#L189-L205 for the existing sandox env config. The proposed esy.sandboxEnv key is a mechanism to override this environment.