Merge changes I29e66ad8,I77ea62cd,I7cd58d92 into main

* changes:
  treewide: make more settings conditionally available
  libstore/build: only send overridden settings to the build hook
  treewide: consistently mark overridden settings as such

src/libexpr/eval-settings.cc

@@ -47,7 +47,7 @@ static Strings parseNixPath(const std::string & s)
 EvalSettings::EvalSettings()
 {
     auto var = getEnv("NIX_PATH");
-    if (var) nixPath = parseNixPath(*var);
+    if (var) nixPath.setDefault(parseNixPath(*var));
 }
 
 Strings EvalSettings::getDefaultNixPath()