spectral: fix secretary

This should fix the benchmark by:
- Not importing `IOExts`.
- Using `randomRs` with a predictable seed instead of `randomRIOs` to
  make the runs reproducible (the latter is using global RNG).
- Bumping one of the parameter to make it run for a bit longer (2s
  instead of 0.4s).

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: build & run

Reviewers: bgamari

Differential Revision: https://phabricator.haskell.org/D3328