github: Upgrade the CI to llvm 23.1.0

Besides just bumping to a new version, this also requires tweaking
the arm64ec build configuration. In the new llvm-mingw toolchains,
the arm64ec target supported by the aarch64-w64-mingw32 sysroot
subdirectory - there's no separate arm64ec-w64-mingw32 sysroot
any longer.

This means that we need to install the updated build into this
directory. If we create a new arm64ec-w64-mingw32 sysroot instead,
the compiler uses that and won't find libunwind and libc++.

Signed-off-by: Martin Storsjö <martin@martin.st>