Fix memory leak reported in main ractor when RUBY_FREE_AT_EXIT

STACK OF 1 INSTANCE OF 'ROOT LEAK: ':
6 dyld 0x1840e20e0 start + 2360
5 miniruby 0x1006796c8 main + 88 main.c:62
4 miniruby 0x10072f4a4 ruby_init + 16 eval.c:99
3 miniruby 0x10072f328 ruby_setup + 104 eval.c:81
2 miniruby 0x1008d08c0 Init_BareVM + 508 vm.c:4276
1 miniruby 0x1007f8944 rb_ractor_main_alloc + 76 ractor.c:2034
0 libsystem_malloc.dylib 0x1842a4cac _malloc_zone_calloc_instrumented_or_legacy + 128¶

1 (96 bytes) ROOT LEAK: <calloc in rb_ractor_main_alloc 0x1347075d0> [96]