[R] `R_existsVarInFrame` isn't available earlier than R 4.2

[jonkeane]

Describe the bug, including details regarding any error messages, version, and platform.

In #43173 we switched to using R_existsVarInFrame instead of Rf_findVarInFrame3, but R_existsVarInFrame isn't available before R 4.2, so we will need to do something to switch on this.

Component(s)

R

[jonkeane]

This is the cause of the r binary build failures at https://github.com/ursacomputing/crossbow/actions/runs/9836224882/job/27151883727

[jonkeane]

Issue resolved by pull request 43243
#43243