generate.py fails without --overwrite

[habernal]

Repro:

$ ./bin/generate examples/sigdial --proceedings
[...]
Traceback (most recent call last):
  File "/home/habi/PycharmProjects/aclpub2/./bin/generate", line 50, in <module>
    generate_proceedings(args.path, args.overwrite, args.outdir, args.nopax, args.frontmatter)
  File "/home/habi/PycharmProjects/aclpub2/aclpub2/generate.py", line 27, in generate_proceedings
    raise Exception(
Exception: Build directory build is not empty, and the overwrite flag is false.

even if the built dir did not exist.

$ python --version
Python 3.12.3