Add scikit-build-core CXX compiler detection when no environment variable is set

[mwyau]

A new config field to detect compilers instead of using the sysconfig default was added in scikit-build-core 1.0: scikit-build/scikit-build-core#1377

https://scikit-build-core.readthedocs.io/en/latest/configuration/index.html#selecting-a-compiler

By default, scikit-build-core sets CC/CXX from Python’s sysconfig compiler when they are not already set. If a project’s compiler probes break on that compiler (a conda narrow sysroot, a stale venv gcc, a cross or oneAPI toolchain), list the variable in the env table to suppress that default and let CMake detect the compiler from PATH; an entry that reads from the same name does this without pinning a value:

[tool.scikit-build.env]
CC = { env = "CC" }
CXX = { env = "CXX" }

• Is CC not needed for ducc?
• This change needs to be tested.

Related issue: #54