extra_envs not working for macOS Intel (only) for packages requiring __osx

Over in https://github.com/mne-tools/mne-installers/pull/411 I'm trying out moving some packages to `extra_envs` but I hit an issue on[ macOS Intel only](https://github.com/mne-tools/mne-installers/actions/runs/21548656397/job/62106412967?pr=411#step:9:2723):
```
Solving environment: ...working... failed
LibMambaUnsatisfiableError: Encountered problems while solving:
- nothing provides __osx >=11.0 needed by vtk-base-9.4.2-py310h470be5d_4
- nothing provides __osx >=11.0 needed by qt6-main-6.9.1-h6d97336_2
- nothing provides __osx >=11.0 needed by pyside6-6.9.1-py310hc926345_0
- nothing provides __osx >=11.0 needed by vtk-base-9.4.2-py310h470be5d_4
- nothing provides __osx >=11.0 needed by vtk-base-9.4.2-py310h470be5d_4
```
This is after the base environment is solved for just fine.

Interestingly there is no issue [on arm64](https://github.com/mne-tools/mne-installers/actions/runs/21548656397/job/62106413050?pr=411#step:9:3145). There is also no issue when these packages [are installed in the base environment](https://github.com/mne-tools/mne-installers/actions/runs/21547489568/job/62091155918#step:9:2603) (on a different recent PR).

It's possible that this is some `conda` bug instead. I might need to get my macOS Intel machine out to try it locally.