runtime: fix dead write to exec_start_timestamp

[two-heart]

the write in fd_runtime_pre_execute_check makes more sense, also given early returns

[github-actions]

Performance Measurements ⏳

Suite	Baseline	New	Change
backtest mainnet-368528500-perf per slot	0.051477 s	0.05171 s	0.453% ✅
backtest mainnet-368528500-perf snapshot load	NaN s	NaN s	NaN% ✅
backtest mainnet-368528500-perf total elapsed	51.476764 s	51.709772 s	0.453% ✅
firedancer mem usage with mainnet.toml	1004.29 GiB	1004.29 GiB	0.000% ✅

actually not sure if this is better to remove, i need to think about it a little bit