actually, a 'lightbulb' moment as a workaround....
according to build.sh , the gcc-arm-none-eabi we need, is
perhaps the easy way forward is to follow the 'build it yourself' method described in the linux install guide (?)
and then just point the runtime directly at the directory you build.
no need to mess about with apt at this point.
really this is quite simple, should just be a matter of
git clone https://github.com/axoloti/axoloti
assuming links are still all good, this will download everything needed
then in axoloti which you have previously installed, changed axoloti runtime in preferences to
of course, this assumes the build script its up to date... which may not be true