change the loop variable to `int` to silence warning like
```
/home/kefu/.local/bin/clang++ -DBOOST_NO_CXX98_FUNCTION_BASE -DDEBUG -DDEBUG_LSA_SANITIZER -DFMT_DEPRECATED_OSTREAM -DFMT_SHARED -DSANITIZE -DSCYLLA_BUILD_MODE=debug -DSCYLLA_ENABLE_ERROR_INJECTION -DSEASTAR_API_LEVEL=7 -DSEASTAR_BROKEN_SOURCE_LOCATION -DSEASTAR_DEBUG -DSEASTAR_DEBUG_SHARED_PTR -DSEASTAR_DEFAULT_ALLOCATOR -DSEASTAR_LOGGER_TYPE_STDOUT -DSEASTAR_SCHEDULING_GROUPS_COUNT=16 -DSEASTAR_SHUFFLE_TASK_QUEUE -DSEASTAR_TYPE_ERASE_MORE -DXXH_PRIVATE_API -I/home/kefu/dev/scylladb -I/home/kefu/dev/scylladb/seastar/include -I/home/kefu/dev/scylladb/build/cmake/seastar/gen/include -I/home/kefu/dev/scylladb/build/cmake/gen -Wall -Werror -Wextra -Wno-error=deprecated-declarations -Wimplicit-fallthrough -Wno-c++11-narrowing -Wno-mismatched-tags -Wno-overloaded-virtual -Wno-unsupported-friend -Wno-unused-parameter -Wno-missing-field-initializers -Wno-deprecated-copy -Wno-ignored-qualifiers -march=westmere -Og -g -gz -std=gnu++20 -fvisibility=hidden -U_FORTIFY_SOURCE -DSEASTAR_SSTRING -Wno-error=unused-result "-Wno-error=#warnings" -fstack-clash-protection -fsanitize=address -fsanitize=undefined -fno-sanitize=vptr -MD -MT tools/CMakeFiles/tools.dir/scylla-nodetool.cc.o -MF tools/CMakeFiles/tools.dir/scylla-nodetool.cc.o.d -o tools/CMakeFiles/tools.dir/scylla-nodetool.cc.o -c /home/kefu/dev/scylladb/tools/scylla-nodetool.cc
/home/kefu/dev/scylladb/tools/scylla-nodetool.cc:215:28: error: comparison of integers of different signs: 'unsigned int' and 'int' [-Werror,-Wsign-compare]
for (unsigned i = 0; i < argc; ++i) {
~ ^ ~~~~
```
`i` is used as the index in a plain C-style array, it's perfectly fine
to use a signed integer as index in this case. as per C++ standard,
> The expression E1[E2] is identical (by definition) to *((E1)+(E2))
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#15431
6c75dc4be8