No global settings in libnixfetchers and libnixflake

Progress on #5638

There are still a global fetcher and eval settings, but they are pushed
down into `libnixcmd`, which is a lot less bad a place for this sort of
thing.

Continuing process pioneered in
52bfccf8d8.

src/libcmd/command.cc

@@ -132,7 +132,7 @@ ref<EvalState> EvalCommand::getEvalState()
             #else
             std::make_shared<EvalState>(
             #endif
-                lookupPath, getEvalStore(), evalSettings, getStore())
+                lookupPath, getEvalStore(), fetchSettings, evalSettings, getStore())
             ;
 
         evalState->repair = repair;