add pin-kernel option

rpi/default.nix

@@ -8,6 +8,15 @@ in
 
   options = with lib; {
     raspberry-pi-nix = {
+      pin-kernel = {
+        enable = mkOption {
+          default = true;
+          type = types.bool;
+          description = ''
+            Whether to pin the kernel to the latest cachix build.
+          '';
+        };
+      };
       firmware-migration-service = {
         enable = mkOption {
           default = true;
@@ -275,7 +284,10 @@ in
       # This pin is not necessary, it would be fine to replace it with
       # `pkgs.rpi-kernels.latest.kernel`. It is helpful to ensure
       # cache hits for kernel builds though.
-      kernelPackages = pinned.linuxPackagesFor (pinned.rpi-kernels.latest.kernel);
+      kernelPackages =
+        if cfg.pin-kernel.enable
+        then pinned.linuxPackagesFor (pinned.rpi-kernels.latest.kernel)
+        else pkgs.linuxPackagesFor (pkgs.rpi-kernels.latest.kernel);
 
       loader = {
         grub.enable = lib.mkDefault false;