Fix const qualifier error in engine_print.c for GCC 14+ compatibility

[buresu]

Change 'c' variable to const char* to fix -Werror=discarded-qualifiers in validateFloatFormat function. GCC 14+ introduced stricter type checking defaults, flagging this assignment as an error.

This PR fixes

mujoco/src/engine/engine_print.c:484:12: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]  484 |   while (c = strchr(flag_characters, float_format[cur_idx]), c != NULL) {

Tested on

• Linux: GCC 15.2.1 (x64)
• Windows: MSVC 19.50.35723 (x64)
• macOS: Apple Clang 17.0.0 (clang-1700.6.3.2) (arm64)

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