nixos-generate-config: detect CPU governor

* cpu-freq: Try powersave if ondemand is not available

* Revert "cpu-freq: Try powersave if ondemand is not available"

This reverts commit 4dc56db37e32dcfecd667ebbf88263e47b296097.
Consult available scaling governors; for freshly generated configs, this provides a better experience than relying on a default that might not work everywhere.