ci: Disable testing for Windows MSYS2 mingw64 clang

ci: Disable testing for Windows MSYS2 mingw64 clang

Clang warns about doctest's include of ciso646:

    In file included from D:/a/ccache/ccache/unittest/main.cpp:26:
    In file included from D:/a/ccache/ccache/build/_deps/doctest-src/doctest/doctest.h:499:
    D:/a/_temp/msys64/mingw64/include/c++/15.1.0/ciso646:46:4: error: "<ciso646> is deprecated in C++17, use <version> to detect implementation-specific macros" [-Werror,-W#warnings]

       46 | #  warning "<ciso646> is deprecated in C++17, use <version> to detect implementation-specific macros"

          |    ^

    1 error generated.

See also doctest issue 900.