Set UV_CONSTRAINT and UV_OVERRIDE for better compat with keras-hub and tensorflow-text

[t-kalinowski]

There is a minor patchwork of incompatibilities to avoid between keras-hub, tensorflow-text, and tensorflow-cpu.

This PR has us set UV_CONSTRAINT and UV_OVERRIDE to ensure that the following simple usage "just works":

library(keras3)
use_backend(ANY, gpu = ANY)

py_require(c("keras-hub", "tensorflow-text"))
keras_hub <- import("keras_hub")
...