GH Actions: Fix conda-forge-maximal

[mkoeppe]

https://github.com/mkoeppe/sage/actions/runs/3519527768/jobs/5899570739

Various conflicts - probably because of the _sagemath dummy package

Depends on #34777

Component: porting

Issue created by migration from https://trac.sagemath.org/ticket/34777

[mkoeppe]

Dependencies: #34777