iso-image: make $ARCH shorter

we use stdenv.hostPlatform.uname.processor, which I believe is just like
`uname -p`.

Example values:
```
(import <nixpkgs> { system = "x86_64-linux"; }).stdenv.hostPlatform.uname.processor
"x86_64"

(import <nixpkgs> { system = "aarch64-linux"; }).stdenv.hostPlatform.uname.processor
aarch64

(import <nixpkgs> { system = "armv7l-linux"; }).stdenv.hostPlatform.uname.processor
"armv7l"
```

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

@@ -427,7 +427,7 @@ in
 
     isoImage.volumeID = mkOption {
       # nixos-$EDITION-$RELEASE-$ARCH
-      default = "nixos${optionalString (config.isoImage.edition != "") "-${config.isoImage.edition}"}-${config.system.nixos.release}-${pkgs.stdenv.hostPlatform.system}";
+      default = "nixos${optionalString (config.isoImage.edition != "") "-${config.isoImage.edition}"}-${config.system.nixos.release}-${pkgs.stdenv.hostPlatform.uname.processor}";
       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