Colab: fix pip install for nightlies

[jameslamb]

Fixes #104

While testing the RAPIDS deployment docs (rapidsai/deployment#434), I tried to follow the Colab instructions for installing RAPIDS libraries with pip.

Found that the script that does that is broken... it has a hard-coded upper bound pin in the set of requirements generated when you ask for nightlies to be installed, resulting in impossible requirements like cudf>=24.10.0a0,<=24.8.

This fixes that, and puts the "latest" and "nightly" versions into module-level constants to make such mistakes less likely in the future.

Notes for Reviewers

How I tested this

Opened the RAPIDS pip colab notebook at https://nvda.ws/3XEO6hK.

Pointed the install lines at my branch here:

!git clone https://github.com/jameslamb/rapidsai-csp-utils.git --branch fix/pip-install
!python rapidsai-csp-utils/colab/pip-install.py nightlies

Ran the rest of the notebook and saw it successfully install the expected versions of libraries.

!pip freeze | grep -E '.*cu.*'

Importing cuml failed, will open a separate issue for that and link it here. Nevermind, just a mix of versions. Everything ran successfully in a clean session!