executor: Allow inheriting env vars if on host or in Docker

Created by: mrnugget

When executing src as part of a batch spec execution we need PATH and HOME to be set so that we can create temp/caching directories and find git in the PATH.

This works without anything from our side when executing src inside Firecracker since ignite exec will SSH into the VM and env vars get.

But when executing src on the host (as is the case when running the executor on macOS) then its environment only contains the values we set, since that's how Go's exec.Command works: if cmd.Env is nil then the command inherits the environment, otherwise only the given values are set.

What this PR attempts is to introduce an InheritLocalEnv option that contains a list of environment variables that the given command should inherit from its environment, but only outside Firecracker.

That exception makes the code a bit ugly, since there's shared code between the Docker and the Firecracker runner and I had to introduce an exception.

But I'm also not sure how to not make it a special case.

Ideas welcome!