Make `run_eval()` targets friendly

[mccarthy-m-g]

If you try to run run_eval() in a targets pipeline with the default settings you will currently get a nebulous error message:

Error:
! Error in tar_make():
  should not be called with handlers on the stack
  See https://books.ropensci.org/targets/debugging.html

You can resolve this by turning off progress bars. We could consider changing the default for this from TRUE to TRUE if interactive (targets is always non-interactive) to resolve this. It may also be specific to using progressr, in which case this might self-resolve if we do #23.