devtools::check() in vignette causes R CMD check to fail.

[dewittpe]

For some reason having the devtools::check in the export vignette results in a R CMD check --as-cran failure, but R CMD check will pass.

To reproduce the error:

git checkout 6c88ae3
make check          # Status Ok
make check-as-cran  # Status 1 Warning, 1 Note

The warning is:

* checking re-building of vignette outputs ... WARNING
Error(s) in re-building vignettes:
  ...
--- re-building ‘api.Rmd’ using rmarkdown
--- finished re-building ‘api.Rmd’

--- re-building ‘export.Rmd’ using rmarkdown
Quitting from lines 128-130 (export.Rmd) 
Error: processing vignette 'export.Rmd' failed with diagnostics:
R CMD check found WARNINGs
--- failed re-building ‘export.Rmd’

SUMMARY: processing the following file failed:
  ‘export.Rmd’

Error: Vignette re-building failed.
Execution halted

The lines causing the issue:

head -n 130 vignettes/export.Rmd 

  ```{r warning = FALSE}
  check <- devtools::check(pkg = prj_dir, quiet = TRUE, cran = FALSE)
  check
  ```