StorePathsCommand: Don't build installables

On second though this was annoying. E.g. "nix log nixpkgs.hello" would
build/download Hello first, even though the log can be fetched
directly from the binary cache.

May need to revisit this.

src/nix/build.cc

@@ -23,7 +23,7 @@ struct CmdBuild : MixDryRun, InstallablesCommand
 
     void run(ref<Store> store) override
     {
-        auto paths = buildInstallables(store, dryRun);
+        auto paths = toStorePaths(store, dryRun ? DryRun : Build);
 
         printInfo("build result: %s", showPaths(paths));
     }