CI: update and fix the iOS 26 / macOS 26 SDK toolchains

[NeroBurner]

Currently we have the osx-15-4-arch-universal2-cxx17.cmake toolchain generated with the following contents:

# toolchain file generated by set_matrix.py, do not modify!
set(CMAKE_OSX_DEPLOYMENT_TARGET "15.4" CACHE STRING "Minimum OS X deployment version" FORCE)
set(CMAKE_OSX_ARCHITECTURES "arm64;x86_64" CACHE STRING "OS X Target Architectures" FORCE)
set(CMAKE_C_COMPILER "clang" CACHE STRING "C compiler" FORCE)
set(CMAKE_CXX_COMPILER "clang++" CACHE STRING "C++ compiler" FORCE)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

In #815 (comment) @olliwang states

I also have some fixes for other packages built with the latest iOS 26 / macOS 26 SDK

and

I submitted these pull requests because the current versions cause build errors with the newer SDK, which was officially released this month.

So I thought we should probably update our toolchains to a more recent version (maybe even the most recent now available)

And maybe fix the toolchains (I've no Mac so I'm bad at debugging related issues)