Disable allowUnfree by default

Fixes #2134.
Replace $HYDRA_DISALLOW_UNFREE with $NIXPKGS_ALLOW_UNFREE

We've never used this environment variable in Hydra (except for a few
days). This is also more consistent with $NIXPKGS_ALLOW_BROKEN.
Add a flag ‘config.allowBroken’
Die tabs die