should we use `stan = stan_opts(method = "vb")` to make github workflows faster to run?

[avallecam]

more accurate settings that come at the cost of speed

we can make duplicates of all the runs of epinow2 in the tutorial for faster PR workflow runs

[avallecam]

assessment:

• stan = EpiNow2::stan_opts(samples = 1000, chains = 2) take less than 3minutes, is more stable between runs
• stan = EpiNow2::stan_opts(method = "vb") is super fast and useful to verify all inputs are working fine