Add icon recipe based on spack 1.0, nvhpc 25.7, upstream libfabric, new nccl, etc.

[msimberg]

This is the test recipe I've been using to have a newer NCCL available for testing with icon, and since it required a bit of ugliness I thought I'd open this as a reference for when the spack 1.0 transition comes officially for c2sm. I'm not looking to have this merged.

In terms of packages, this contains:

• nvhpc 25.7 (nvhpc 25.9 would contain CUDA 12.9 and 13.0, some testing would be required to make sure 12.9 is used, or that 13.0 works)
• gcc 13 (GHEX, dependency of icon4py, currently does not build with gcc 14)
• nccl 2.28, aws-ofi-nccl 1.17, libfabric 2.3 (these are all required for decent nccl performance; libfabric 2.3 from upstream may also slightly improve performance for MPI or improve stability)

Ugliness:

• I needed to add the concretizer:duplicates:strategy:full option to get the recipe to concretize (default is minimal, see the PR adding support to stackinator for some more details: Concretizer duplicates strategy eth-cscs/stackinator#269); I hope this is a bug that might be resolved in newer spack versions but it's not clear to me yet if that's the case
• The above option means that a lot of packages concretize with nvhpc as the compiler; I had to force many packages to use gcc explicitly
• cdo did not build and since I didn't need it I just disabled it for now
• CC, CXX, and FC are set to nonexistent paths; for now, unset them and use MPICC et al.
• There's no libnvToolsExt.so library in nvhpc 25.7 (build scripts set -lnvToolsExt currently); I think it's not needed but have not tested any profiling

The changes required to the custom packages in the repo directory were minimal. I used upstream spack-packages instead of the spack-c2sm package repo.

In case anyone wants to use the environment I have a squashfs file in /capstor/store/cscs/cscs/csstaff/simbergm/icon-spack-1.0-test.squashfs.