fix NIX_PATH for real (#11079)

* fix NIX_PATH overriding

- test restricted evaluation
- test precedence for setting the search path

Co-authored-by: Robert Hensing <robert@roberthensing.nl>
Co-authored-by: John Ericson <git@JohnEricson.me>

src/libexpr/eval-gc.cc

@@ -1,5 +1,7 @@
 #include "error.hh"
 #include "environment-variables.hh"
+#include "eval-settings.hh"
+#include "config-global.hh"
 #include "serialise.hh"
 #include "eval-gc.hh"
 
@@ -230,6 +232,12 @@ void initGC()
     gcCyclesAfterInit = GC_get_gc_no();
 #endif
 
+    // NIX_PATH must override the regular setting
+    // See the comment in applyConfig
+    if (auto nixPathEnv = getEnv("NIX_PATH")) {
+        globalConfig.set("nix-path", concatStringsSep(" ", EvalSettings::parseNixPath(nixPathEnv.value())));
+    }
+
     gcInitialised = true;
 }