Fix CI: useless cast in the Zmij digit writer and snprintf truncation

- ci_test_gcc (Linux x86-64): static_cast<std::size_t>(d.significand % 100)
  was a useless cast (a std::uint64_t prvalue, the same type as
  std::size_t there); cast a named variable instead.
- ci_test_gcc: -Werror=format-truncation for snprintf("%.*e") in
  unit-to_chars.cpp, whose precision GCC cannot bound; write the
  neighboring decimal with a stream (classic locale, std::scientific),
  which gives the same text.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/conversions/to_chars.hpp

@@ -983,7 +983,8 @@ inline void shortest_digits(char* buf, int& len, int& decimal_exponent, double v
     std::size_t n = digits.size();
     while (d.significand >= 100)
     {
-        const auto i = static_cast<std::size_t>(d.significand % 100) * 2;
+        const std::uint64_t two_digits = d.significand % 100; // a variable: GCC calls a cast of the remainder useless where std::uint64_t is std::size_t
+        const auto i = static_cast<std::size_t>(two_digits) * 2;
         d.significand /= 100;
         n -= 2;
         digits[n] = pairs[i];