Clarify caller_env is from rlang

[MichaelChirico]

Just bit by this:

https://github.com/r-lib/lintr/actions/runs/19476159740/job/55736370297?pr=2967

And first guessed testthat::caller_env(). I think this little nudge is enough. We could also move it to the formals of expect_length_ below.

[hadley]

You can also use parent.frame() if you want to avoid an explict rlang dep.