Add a name to the stores

So that it can be printed by `nix describe-stores`

src/libstore/local-store.hh

@@ -37,6 +37,8 @@ struct LocalStoreConfig : virtual LocalFSStoreConfig
     Setting<bool> requireSigs{(StoreConfig*) this,
         settings.requireSigs,
         "require-sigs", "whether store paths should have a trusted signature on import"};
+
+    const std::string name() override { return "Local Store"; }
 };