drop the fast path for enableNativeCode == true

[chayleaf]

This caused issues when Nix was used as a library, because eval settings are only present in the binary.

For example, Hydra (among other things) errored out like this:

error: could not dynamically open plugin file '"/nix/store/2jr1wzy4ph2pv0h9fzpw1375ywbh6psl-nix-plugins-15.0.0/lib/nix/plugins/libnix-extra-builtins.so"':  /nix/store/2jr1wzy4ph2pv0h9fzpw1375ywbh6psl-nix-plugins-15.0.0/lib/nix/plugins/libnix-extra-builtins.so: undefined symbol: _ZN3nix12evalSettingsE

[shlevy]

Hmm why is there Nix evaluation happening without eval settings? I don’t think we can rely on the evaluator not hitting evalSettings even if nix-plugins avoids it. Seems like a bug in whatever program is driving this.