Use variables for iso image creation

[mojca]

Rationale for this change:

I wanted to create a 64-bit image using a VM inside macOS running on a M1 chip (the image creation is super fast compared to the timings cited in your README), and I wanted to use the latest RaspiOS for that. In order to do that I needed to use a different image, but changing the gazillion of repetitive names is super error prone.

This minor change makes it much easier to switch to a different architecture and different OS version (that is: it requires a lot less changes).

Without the last compression step and using arm64, bullseye, 2022-09-22, 2022-09-26 , the script needs 2,5 minutes to complete on the oldest & cheapest mac mini with M1.