Workaround build error with clang/libc++ in agi::fs::path

[driver1998]

The std::basic_fstream implementation in libc++ does not allow constructing with a derived class of std::filesystem::path like agi::fs::path. (There is a is_same_v constraint.)

Workaround by getting its underly string manually. Fixes building with LLVM based MinGW environments, including Windows on ARM.

This kind of defeats the purpose of agi::fs::path, therefore currently sent as draft.