Set cwd when running input validators

[Matistjati]

Closes #336. I think that this is the right solution- changing the limit ourselves adds more noise to the codebase as opposed to just masking it by dumping the core in the temporary working directory.

Doesn't result in a statistically significant slowdown.

matistjati@DESKTOP-BJV27M6:~/po/swedish-olympiad-2026/lager2$ ulimit -c unlimited
matistjati@DESKTOP-BJV27M6:~/po/swedish-olympiad-2026/lager2$ time ~/kattis/problemtools/bin/verifyproblem.sh fredrikspizzeria -p data
Loading problem fredrikspizzeria
Checking data
fredrikspizzeria tested: 0 errors, 0 warnings

real    0m25.040s
user    0m20.809s
sys     0m2.376s
matistjati@DESKTOP-BJV27M6:~/po/swedish-olympiad-2026/lager2$ ulimit -c 0
matistjati@DESKTOP-BJV27M6:~/po/swedish-olympiad-2026/lager2$ time ~/kattis/problemtools/bin/verifyproblem.sh fredrikspizzeria -p data
Loading problem fredrikspizzeria
Checking data
fredrikspizzeria tested: 0 errors, 0 warnings

real    0m24.472s
user    0m20.102s
sys     0m2.428s