[bug] Building Conan itself fails

### Describe the bug

Hello,

for historical reasons we build Conan from source on all of our required architectures. Recently the builds started to fail on macOS.

I was able to create a [reproducer](https://github.com/gjasny/conan-build-issue) and have it fail on a GitHub Actions Runner [here](https://github.com/gjasny/conan-build-issue/actions/runs/14752217285/job/41412128454) with:

```
self.initialize_options()
/Users/runner/work/conan-build-issue/conan-build-issue/_pyenv/bin/python3: No module named pip
```

Some observations:
* It only fails on macOS
* Also older Conan versions that used to succeed building now fail (which makes me suspect some updated Python dependency on Pypi side)

Could you please take a look?

Thanks,
Gregor

PS: Are the build recipes for the official Conan releases available somewhere?

### How to reproduce it

Clone https://github.com/gjasny/conan-build-issue and run `build.sh` locally.