The C API provides nix_eval_state_builder_load as a separate function
to allow controlling whether settings are loaded from the environment.
Add load_ambient_settings() method to expose this control, with the
default being to load them (needed in some situations to allow path
coercion to work).
Without this call, settings from the global Nix configuration are never
loaded, leaving readOnlyMode = true (the default). This prevents Nix
from adding paths to the store during evaluation, causing errors like:
error: path '/some/local/path' does not exist
This fix loads global settings before creating the EvalState, enabling
path coercion for local file references.
