fix brms::prior in test

[gravesti]

https://statistical-engineering.pages.roche.com/-/systems-integration-tests/-/jobs/59706082/artifacts/public/logs/check-jmpost.html

══ Failed tests ════════════════════════════════════════════════════════════════
  ── Error ('test-brms_compare.R:104:5'): jmpost and brms get similar loo for longitudinal models ──
  Error in `prior("normal(log(60), 0.6)", nlpar = "b")`: could not find function "prior"
  Backtrace:
      ▆
   1. └─brms::brm(...) at test-brms_compare.R:104:5
   2.   └─brms:::.validate_prior(prior, bterms = bterms, data = data, sample_prior = sample_prior)