Fix cppcheck warning

[nlohmann]

The serialization of floating-point numbers is handled in two code paths. If number_float_t is an IEEE-754 single or double precision number, the Grisu2 algorithm is used. Otherwise, the serialization is done via std::snprintf. By updating cppcheck to 1.5.1, it warns that the used specifier "%.*g" only works with double.

This PR adds a conversion to double in the std::snprintf call to silence this warning.

Closes #4755

[coveralls]

coverage: 99.188%. remained the same
when pulling dfb9eb6 on issue4755-cppcheck-warning
into 51a77f1 on develop.

[github-actions]

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