Feature request: Export `on_cran()`

I have a package that has parallel operations by default. That's handy for testing locally, but it fails the CRAN test that it uses >2 cores by default. I would like to gate the settings on CRAN to use something like:

```
if (on_cran()) {
   set_cores(1)
}
```

Currently, I've just copied the code for `on_cran()` to `on_cran_local()` so that I can use it.