`x setup` doesn't work if `./build` is a symlink to a non-existent directory

[WaffleLapkin]

Summary

x setup crashes when ./build is a symlink to a non-existent directory.

Command used

x setup

Expected behaviour

x should create the directory linked to by the symlink.

Actual behaviour

: ~/p/rust-lib (+ sxq 470 empty); x setup
Traceback (most recent call last):
  File "/home/wffl/prog/rust-lib/x.py", line 53, in <module>
    bootstrap.main()
  File "/home/wffl/prog/rust-lib/src/bootstrap/bootstrap.py", line 1339, in main
    bootstrap(args)
  File "/home/wffl/prog/rust-lib/src/bootstrap/bootstrap.py", line 1301, in bootstrap
    os.makedirs(build.build_dir)
  File "<frozen os>", line 225, in makedirs
FileExistsError: [Errno 17] File exists: '/home/wffl/prog/rust-lib/build'

Operating system

NixOS

HEAD

30f168e