en_US.UTF-8 should not be check

[flagarde]

setting en_US.UTF-8 would create strange behaviour for system with other default locale.

CLI11/include/CLI/impl/Encoding_inl.hpp

Line 36 in 2d79205

static const std::array<const char *, 3> unicode_locales{{"C.UTF-8", "en_US.UTF-8", ".UTF-8"}};