Build process is broken

I reported this in June #879 (comment)

Steps to reproduce the bug

I tried building v15 myself but it wouldn't complete in ways I do not understand.

$ grunt desktop-mac
Loading "node_webkit_builder.js" tasks...ERROR
>> TypeError: os.tmpDir is not a function
Warning: Task "nwjs:macos" not found. Use --force to continue.

Aborted due to warnings.

Environment details

• operating system: macOS 10.14.6