Make sure that CUDA(-dependent) builds are always done in an accelerator prefix

[bedroge]

I accidentally triggered a CUDA+UCX-CUDA build for zen2 without accelerator target in https://github.com/EESSI/software-layer/pull/817#issuecomment-2490279284, and this worked fine, so I almost overlooked that I overlooked that I forgot to specify the accelerator target. It would be nice to have some check (CI, hook?) in place that prevents us from doing that, i.e. to make sure that these CUDA builds always are done in an accelerator prefix.

[ocaisa]

We should be able to do that via an EasyBuild hook (should be just another piece of logic alongside what is currently there when checking CUDA dependencies)

[ocaisa]

Should be a matter of checking the install path at https://github.com/EESSI/software-layer/blob/2023.06-software.eessi.io/eb_hooks.py#L945