[Python] Venv tweaks based on testing in Sandboxes

[Lagoja]

Summary

A few small tweaks after testing in Sandboxes

1. Move the STATE_CHECK to after we verify that a VENV exists and that it's properly formatted. This ensures that we can fix scenarios where the venv was deleted or broken, without excessively prompting the user
2. Add a UV_PYTHON environment variable to ensure that users who install uv don't accidentally overwrite the Python version in their devbox.json

How was it tested?

1. Open the python-flask Jetify Example in Sandboxes
2. Test .venv is setup correctly with devbox shell
3. Delete the .venv dir, verify that re-running devbox shell creates it
4. Install uv (devbox add uv)
5. Verify that uv sync and other commands don't override Devbox Python