Removing -DSlicer_USE_PYTHONQT=OFF made the build past that point, so Python is necessary to build Slicer.
Past that point I run into
CMake Error at Libs/vtkTeem/CMakeLists.txt:151 (vtkMacroKitPythonWrap):
Unknown CMake command "vtkMacroKitPythonWrap".
at: ENH: Add GitHub actions build, test workflow file for CI · jhlegarreta/SlicerDMRI@a20943a · GitHub
Which was solved following:
Unknown CMake command "vtkMacroKitPythonWrap" in snapshot release for vtkTeem - #4 by cpinter
Slicer looks to be building now:
ENH: Add GitHub actions build, test workflow file for CI · jhlegarreta/SlicerDMRI@35e3b27 · GitHub