nixos/powerManagement: set `cpuFreqGovernor` with `mkOptionDefault`

`nixos-generate-config` detects the `cpuFreqGovernor` suited best for my
machine, e.g. `powerManagement.cpuFreqGovernor = lib.mkDefault "powersave";`.

However the `powerManagement` module sets a sensitive default for
`cpuFreqGovernor` using `mkDefault` to avoid breackage with older
setups. Since 140ac2f1 the `hardware-configuration.nix` sets the
gorvernor with `mkDefault` as well which causes evaluation errors if the
powermanagement module is enabled:

```
error: The unique option `powerManagement.cpuFreqGovernor' is defined multiple times, in `/home/ma27/Projects/nixos-config/hardware-configuration.nix' and `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/nixos/modules/config/power-management.nix'.
```

Using `mkOptionDefault` rather than `mkDefault` in the powermanagement
module fixes this issue as it decreases the priority of the module and
prefers the value set in `hardware-configuration.nix`.

I have confirmed the change using the following VM declaration:

```
{
  cpuFreq = { lib, ... }: {
    powerManagement.cpuFreqGovernor = lib.mkDefault "powersave";
    powerManagement.enable = true;
  };
}
```

1 files changed, 1 insertions, 1 deletions

diff --git a/nixos/modules/config/power-management.nix b/nixos/modules/config/power-management.nix
index a4a4d6e1a6af..7a5ce6ee5b99 100644
--- a/nixos/modules/config/power-management.nix
+++ b/nixos/modules/config/power-management.nix
@@ -70,7 +70,7 @@ in
   config = mkIf cfg.enable {
 
     # Leftover for old setups, should be set by nixos-generate-config now
-    powerManagement.cpuFreqGovernor = mkDefault "ondemand";
+    powerManagement.cpuFreqGovernor = mkOptionDefault "ondemand";
 
     systemd.targets.post-resume = {
       description = "Post-Resume Actions";