bug: running fuel-core as a binary sensitive to env variables

[segfault-magnet]

The fuel-core started reading environment variables recently. This introduces potential issues for our users.

One example is they use a generic PERIOD env variable to set the poa-interval which causes problems. Zsh users which use zplug are going to have PERIOD set in their environment and then fuel-core is going to have issues with it.

The best approach might just be to either switch off the environment reading by the fuel-core or make the environment empty for the spawned fuel-core process.