Set uv UV_PYTHON_INSTALL_DIR explicitly

[sebastianliebscher]

What does this PR do?

In recent commits, uv was introduced. When installing Python via uv python install it defaults to ~/.local/share/uv/python. Since the user context in the base stage of the Dockerfile is root, the default path resolves to /root/.local/share/uv/python. Unfortunately, this directory is not accessible to non-root users in unprivileged containers, e.g.

docker run --rm --entrypoint bash -it --user 1000 ghcr.io/huggingface/text-generation-inference:3.2.3 -c "ls /root/.local/share/uv"
ls: cannot access '/root/.local/share/uv': Permission denied

This PR fixes this by explicitly setting the UV_PYTHON_INSTALL_DIR variable to /usr/share/uv which is accessible for non-root users.

Fixes #3082

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@Narsil

[sebastianliebscher]

@Narsil any chance this will get a review? Let me know if there's anything I'm missing.