Avoid a call to derivationFromPath()

This doesn't work in read-only mode, ensuring that operations like

  nix path-info --store https://cache.nixos.org -S nixpkgs.hello

(asking for the closure size of nixpkgs.hello in cache.nixos.org) work
when nixpkgs.hello doesn't exist in the local store.

src/nix/log.cc

@@ -28,7 +28,8 @@ struct CmdLog : InstallablesCommand
         subs.push_front(store);
 
         for (auto & inst : installables) {
-            for (auto & path : inst->toBuildable()) {
+            for (auto & b : inst->toBuildable()) {
+                auto path = b.second.drvPath != "" ? b.second.drvPath : b.first;
                 bool found = false;
                 for (auto & sub : subs) {
                     auto log = sub->getBuildLog(path);