Forward the whole Nix config to the post-build-hook

Fill `NIX_CONFIG` with the value of the current Nix configuration before
calling the post-build-hook.
That way the whole configuration (including the possible
`experimental-features`, a possibly `--store` option or whatever) will
be made available to the hook

src/libstore/build/derivation-goal.cc

@@ -757,6 +757,7 @@ void runPostBuildHook(
 
     hookEnvironment.emplace("DRV_PATH", store.printStorePath(drvPath));
     hookEnvironment.emplace("OUT_PATHS", chomp(concatStringsSep(" ", store.printStorePathSet(outputPaths))));
+    hookEnvironment.emplace("NIX_CONFIG", globalConfig.toKeyValue());
 
     RunOptions opts(settings.postBuildHook, {});
     opts.environment = hookEnvironment;