Currently, base images are installing / updating pip and uv from the variant-specific Pulp PyPI, e.g. "cuda12.9-ubi9" image uses the "cuda12.9-ubi9" Pulp index. This is causing some problems:

• some variants do not have a public Pulp index, yet, .e.g TPU and Spyre. The base images for these platforms don't have uv.
• bootstrapping a new variant is awkward, because a base image cannot be build until the Pulp index has been filled and released.

In order to work around the issue, change the install script to pull "pip" and "uv" from a common index. For now, let's use the "cpu-ubi9" image. In the future, introduce an unversioned index for core packages.