Customise list of packages in revdep check in `use_release_issue()`

[hadley]

e.g. waldo needs pkgs <- union(devtools::revdep("testthat"), devtools::revdep("waldo")) because most of the uses are via testthat.

[hadley]

With r-lib/revdepcheck#352, this becomes revdepcheck::cloud_check(revdep_packages = revdepcheck::cran_revdeps(c("testthat", "waldo"))

[hadley]

And now revdepcheck::cloud_check(extra_revdeps = "testthat").