worldofpeace df8c30fa25 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"
```
2020-04-09 20:10:01 -04:00
..
2020-04-09 20:10:01 -04:00

*** NixOS ***

NixOS is a Linux distribution based on the purely functional package
management system Nix.  More information can be found at
http://nixos.org/nixos and in the manual in doc/manual.