Cannot install when PYTHONSAFEPATH=true

[jasonk]

Describe the bug
I was having problems trying to install into a devcontainer (using the mcr.microsoft.com/devcontainers/python:3.13-bullseye image). I tracked it down to having PYTHONSAFEPATH set.

To Reproduce

> export PYTHONSAFEPATH=true
> uv add ulid-transform
Resolved 160 packages in 100ms
  × Failed to build `ulid-transform==1.4.0`
  ├─▶ The build backend returned an error
  ╰─▶ Call to `poetry.core.masonry.api.build_wheel` failed (exit status: 1)

      [stderr]
      A setup.py file already exists. Using it.
      Traceback (most recent call last):
        File
      "/home/vscode/.cache/uv/sdists-v9/pypi/ulid-transform/1.4.0/rKaEPXP96zhbpBax9TOp4/src/setup.py",
      line 28, in <module>
          from build_ext import *
      ModuleNotFoundError: No module named 'build_ext

Trying it again without having that set works, because setup.py is importing build_ext.py in a way that only works when not running with safe path enabled..

> export PYTHONSAFEPATH=
> uv add ulid-transform
...installs successfully...

Additional context
Reporting it here mostly so that if anybody else runs into this they'll be able to find out why...