Fix output dir creation

Fix output dir creation

The cache dir is created inside the output dir, so we need to create the
output dir at least before that. Fix the following error:

$ sudo mkosi -d fedora -r 29 -t directory -O bla --default /dev/null

Traceback (most recent call last):
  File "/bin/mkosi", line 3767, in <module>
    main()
  File "/bin/mkosi", line 3757, in main
    build_stuff(args)
  File "/bin/mkosi", line 3573, in build_stuff
    setup_package_cache(args)
  File "/bin/mkosi", line 2412, in setup_package_cache
    d = tempfile.TemporaryDirectory(dir=os.path.dirname(args.output), prefix=".mkosi-")
  File "/usr/lib64/python3.7/tempfile.py", line 788, in __init__
    self.name = mkdtemp(suffix, prefix, dir)
  File "/usr/lib64/python3.7/tempfile.py", line 366, in mkdtemp
    _os.mkdir(file, 0o700)
FileNotFoundError: [Errno 2] No such file or directory: '/home/lucas/p/mkosi/bla/.mkosi-l3oxfchg'