`addEta()` fails to correctly add IIV when there is a covariate

[billdenney]

The below should have automatically detected the IIV to be on the right hand side next to lcl:

library(nlmixr2lib)
library(rxode2)
modBase <-
  readModelDb("PK_1cmt") %>%
  model(
    cl <- exp(lcl + allo_cl*WT)
  ) %>%
  ini(
    allo_cl <- 0.75
  )

modBase %>%
  addEta("cl")