No global eval settings in libnixexpr

Progress on #5638

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

src/libexpr/parser-state.hh

@@ -46,6 +46,7 @@ struct ParserState
     PosTable::Origin origin;
     const ref<SourceAccessor> rootFS;
     const Expr::AstSymbols & s;
+    const EvalSettings & settings;
 
     void dupAttr(const AttrPath & attrPath, const PosIdx pos, const PosIdx prevPos);
     void dupAttr(Symbol attr, const PosIdx pos, const PosIdx prevPos);