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/command.cc

@@ -126,13 +126,11 @@ ref<EvalState> EvalCommand::getEvalState()
 {
     if (!evalState) {
         evalState =
+            std::allocate_shared<EvalState>(
             #if HAVE_BOEHMGC
-            std::allocate_shared<EvalState>(traceable_allocator<EvalState>(),
-                lookupPath, getEvalStore(), getStore())
-            #else
-            std::make_shared<EvalState>(
-                lookupPath, getEvalStore(), getStore())
+                traceable_allocator<EvalState>(),
             #endif
+                lookupPath, getEvalStore(), evalSettings, getStore())
             ;
 
         evalState->repair = repair;

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({

src/libcmd/common-eval-args.hh

@@ -12,9 +12,15 @@ namespace nix {
 
 class Store;
 class EvalState;
+struct EvalSettings;
 class Bindings;
 struct SourcePath;
 
+/**
+ * @todo Get rid of global setttings variables
+ */
+extern EvalSettings evalSettings;
+
 struct MixEvalArgs : virtual Args, virtual MixRepair
 {
     static constexpr auto category = "Common evaluation options";