Fix default rlimit_stack value

Fix default rlimit_stack value

The default `rlimit_stack` value was set to 1048576. However, this value is in MiB and so is later multiplied by 1024*1024 in https://github.com/google/nsjail/blob/b3d544d155f5d1543dce1bd3e5327ef41583815a/config.cc#L161-L162 and it ends up as a limit of 1 TB for the stack size.

This PR changes it to 8 MB which is a more sane default or, at least I took it from my virtual machine's ulimits:
```
$ ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 31175
max locked memory       (kbytes, -l) 16384
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 31175
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited
```