Fix build hook error for libstore library users

A library shouldn't require changes to the caller's argument handling,
especially if it doesn't have to, and indeed we don't have to.

This changes the lookup order to prioritize the hardcoded path to nix
if it exists. The static executable still finds itself through /proc
and the like.

tests/test-libstoreconsumer/main.cc

@@ -0,0 +1,45 @@
+#include "globals.hh"
+#include "store-api.hh"
+#include "build-result.hh"
+#include <iostream>
+
+using namespace nix;
+
+int main (int argc, char **argv)
+{
+    try {
+        if (argc != 2) {
+            std::cerr << "Usage: " << argv[0] << " store/path/to/something.drv\n";
+            return 1;
+        }
+
+        std::string drvPath = argv[1];
+
+        initLibStore();
+
+        auto store = nix::openStore();
+
+        // build the derivation
+
+        std::vector<DerivedPath> paths {
+            DerivedPath::Built {
+                .drvPath = store->parseStorePath(drvPath),
+                .outputs = OutputsSpec::Names{"out"}
+            }
+        };
+
+        const auto results = store->buildPathsWithResults(paths, bmNormal, store);
+
+        for (const auto & result : results) {
+            for (const auto & [outputName, realisation] : result.builtOutputs) {
+                std::cout << store->printStorePath(realisation.outPath) << "\n";
+            }
+        }
+
+        return 0;
+
+    } catch (const std::exception & e) {
+        std::cerr << "Error: " << e.what() << "\n";
+        return 1;
+    }
+}