treewide: consistently mark overridden settings as such

Only overridden settings are sent to the daemon, and we're going to do the same
for the build hook to. It needs to be ensured that overridden settings are in
fact consistently marked as such, so that they actually get sent.

Change-Id: I7cd58d925702f86cf2c35ad121eb191ceb62a355

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()