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/libcmd/common-eval-args.cc

@@ -1,6 +1,7 @@
 #include "eval-settings.hh"
 #include "common-eval-args.hh"
 #include "shared.hh"
+#include "config-global.hh"
 #include "filetransfer.hh"
 #include "eval.hh"
 #include "fetchers.hh"
@@ -13,6 +14,12 @@
 
 namespace nix {
 
+EvalSettings evalSettings {
+    settings.readOnlyMode
+};
+
+static GlobalConfig::Register rEvalSettings(&evalSettings);
+
 MixEvalArgs::MixEvalArgs()
 {
     addFlag({