The Bullseye MinGw version (posix variant) of std::filesystem returns an empty path from temp_directory_path(). This is not a problem for now because we use the boost filesystem variant, but it is in the way of migrating ot std::filesystem.
This diffs creates a new fsbridge function to get a temporary directory path that doesn't use temp_directory_path() for windows, preventing the bug.
Note that this function is only used during the execution of the unit tests.

ninja check

Run the windows cross build tests.