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"
```