Find a way to built stacks with "Intel oneAPI builds with ifort" and "Intel oneAPI builds with ifx"

[climbfuji]

Is your feature request related to a problem? Please describe.

We need to be able to build stacks with Intel oneAPI icx+icpx+ifort and at the same time with Intel oneAPI icx+icpx+ifx. So far, I have labor-some solutions for Atlantis and Nautilus, where the oneapi@2024 compiler with ifort is default, and oneapi@2025 with ifx is commented out (and similarly the corresponding mkl, mpi, tbb, runtime packages). Using the latter means removing the entries belonging to 2024 from compilers.yaml and packages_oneapi.yaml, and uncommenting the 2025 entries after creating the environment.

Also, to distinguish these environments better, I call them ue-oneapi-2024.2.1 and ue-oneifx-2025.0.3, for example.

Describe the solution you'd like
The issue is that both compilers are known as oneapi in spack. We could play tricks like making oneifx known to the spack stack create env extension, and then internally copy packages_oneifx.yaml instead of _oneapi.yaml. But we might be able to specify the exact compiler as argument for spack stack create env, in which case this is almost a non-problem (spack stack create env --compiler=oneapi@2025.0.3 --name=ue-oneifx-2025.0.3 ...).

Additional context
This is a temporary problem, until the last stack with oneapi@2024.2.1 using ifort is gone.