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

[gkreitz]

Ideally, we should probably try to follow https://www.kattis.com/problem-package-format/spec/2025-09.html#working-directory here (even if we need to fudge things a bit - as there isn't really a submission here...)

But it's clear progress not to litter the package with core files, thanks! :)