Disable unsat cores in timeout regression (#4713)

Disable unsat cores in timeout regression (#4713)

Fixes a timeout in the nightlies.

One regression times out during unsat core checking. This appears to be random, the subcall to check for unsat-cores is applying sygus-inst in the expected way, however, it struggles to find the right instances.

Furthermore note that we are planning to redo implementation of unsat cores soon (when proof-new is fully merged), so we should revisit this (and all other) regressions with --no-check-unsat-cores when that happens.