iso-image: normalize volumeID

The volumeID will now be in the format of:
nixos-$EDITON-$RELEASE-$ARCH

an example for the minimal image would look like:
nixos-minimal-20.09-x86-64-linux

nixos/modules/installer/cd-dvd/installation-cd-base.nix

@@ -18,8 +18,6 @@ with lib;
   # ISO naming.
   isoImage.isoName = "${config.isoImage.isoBaseName}-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}.iso";
 
-  isoImage.volumeID = substring 0 11 "NIXOS_ISO";
-
   # EFI booting
   isoImage.makeEfiBootable = true;
 

nixos/modules/installer/cd-dvd/installation-cd-graphical-gnome.nix

@@ -7,6 +7,8 @@ with lib;
 {
   imports = [ ./installation-cd-graphical-base.nix ];
 
+  isoImage.edition = "gnome";
+
   services.xserver.desktopManager.gnome3.enable = true;
 
   # Wayland can be problematic for some hardware like Nvidia graphics cards.

nixos/modules/installer/cd-dvd/installation-cd-graphical-plasma5.nix

@@ -8,6 +8,8 @@ with lib;
 {
   imports = [ ./installation-cd-graphical-base.nix ];
 
+  isoImage.edition = "plasma5";
+
   services.xserver = {
     desktopManager.plasma5 = {
       enable = true;

nixos/modules/installer/cd-dvd/installation-cd-minimal.nix

@@ -8,5 +8,7 @@
     [ ./installation-cd-base.nix
     ];
 
+  isoImage.edition = "minimal";
+
   fonts.fontconfig.enable = false;
 }

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -417,8 +417,17 @@ in
       '';
     };
 
+    isoImage.edition = mkOption {
+      default = "";
+      description = ''
+        Specifies which edition string to use in the volume ID of the generated
+        ISO image.
+      '';
+    };
+
     isoImage.volumeID = mkOption {
-      default = "NIXOS_BOOT_CD";
+      # nixos-$EDITION-$RELEASE-$ARCH
+      default = "nixos${optionalString (config.isoImage.edition != "") "-${config.isoImage.edition}"}-${config.system.nixos.release}-${pkgs.stdenv.hostPlatform.system}";
       description = ''
         Specifies the label or volume ID of the generated ISO image.
         Note that the label is used by stage 1 of the boot process to