Fix compiler warnings.

[trivialfis]

• Broadcast string directly.
• Avoid type-erased pointer.

/src/metric/../collective/aggregator.h:46:16:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:95:34: warning: ‘char* __builtin_strncpy(char*, const char*, long 
unsigned int)’ specified bound 1024 equals destination size [-Wstringop-truncation]
   95 |   return __builtin___strncpy_chk (__dest, __src, __len,
      |          ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
   96 |                                   __glibc_objsize (__dest));

[trivialfis]

cc @rongou .

There's an option in xgboost CMake: ENABLE_ALL_WARNINGS, which can be useful from time to time.