nixos: use supportedSystems argument instead of hardcoded list for netboot

The default value for the argument is identical to the hardcoded list,
but using the argument allows to build other netboot images easily.

nixos/release.nix

@@ -138,7 +138,7 @@ in rec {
   # Build the initial ramdisk so Hydra can keep track of its size over time.
   initialRamdisk = buildFromConfig ({ ... }: { }) (config: config.system.build.initialRamdisk);
 
-  netboot = forMatchingSystems [ "x86_64-linux" "aarch64-linux" ] (system: makeNetboot {
+  netboot = forMatchingSystems supportedSystems (system: makeNetboot {
     module = ./modules/installer/netboot/netboot-minimal.nix;
     inherit system;
   });