title: TypeError when set target to build Fuchsia
body: <p>Trying to set a target before building Fuchsia, but getting the following error. 
  I tried converting the <code>sep</code> to byte by using <code>b.startswith(b'sep')</code> then it gives the error.
  So I am just stuck on how to deal with this. I have zero knowledge of python.</p>
  
  <pre><code>prashant@prashant-Lenovo:~/fuchsia$ scripts/fx set x64
  </code></pre>
  
  <p>and the output is : </p>
  
  <pre><code>ERROR at //build/gn/BUILD.gn:67:5: Script returned non-zero exit code.
      exec_script("//build/zircon/list_source_files.py", [], "list lines")
      ^---------- Current dir: /home/prashant/fuchsia/out/x64/ Command: /usr/bin/env -- /home/prashant/fuchsia/build/zircon/list_source_files.py Returned 1. stderr:
  
  Traceback (most recent call last):   File "/home/prashant/fuchsia/build/zircon/list_source_files.py", line 36, in &lt;module&gt;
      sys.exit(main())   File "/home/prashant/fuchsia/build/zircon/list_source_files.py", line 31, in main
      for file in get_files():   File "/home/prashant/fuchsia/build/zircon/list_source_files.py", line 27, in get_files
      return [os.path.join(ZIRCON_ROOT, file) for file in files]   File "/home/prashant/fuchsia/build/zircon/list_source_files.py", line 27, in &lt;listcomp&gt;
      return [os.path.join(ZIRCON_ROOT, file) for file in files]   File "/usr/lib/python3.6/posixpath.py", line 87, in join
      if b.startswith(sep): TypeError: startswith first arg must be bytes or a tuple of bytes, not str
  </code></pre>