Fix longitudinal benchmark

[zliu41]

This was broken by #5371

"The function that receives the environment must use lazy pattern matching": https://hackage.haskell.org/package/criterion-1.6.3.0/docs/Criterion.html#v:env

Otherwise

validation: Criterion atttempted to retrieve a non-existent environment!
	Perhaps you forgot to use lazy pattern matching in a function which
	constructs benchmarks from an environment?
	(see the documentation for `env` for details)