libstdc++ 14's <format> initializes a size_t parameter with -1 (GCC bug 119429), so every std::format call fails ci_test_clang_sanitizer under -fsanitize=integer (test-std-format_cpp20). Exclude only the implicit-integer-sign-change check and only that header via -fsanitize-ignorelist. Signed-off-by: Niels Lohmann <mail@nlohmann.me>