Adding `env_vars` argument to `rhub_check()`?

[thegargiulian]

Prior to v2, it was possible to specify an environmental variable in rhub::check() using the env_vars argument. Similar to #619, I'm running into issues with the arrow installation on MacOS tests, although in my case arrow is a necessary dependency. I'd like to be able to specify that arrow not be built from source as was possible prior to v2 (see discussion in #371).

Thank you!