fix(ci): install python3-venv before creating community GPU staging venv

[zhenshanx-nv]

Problem

Community GPU CI runs python3 -m venv directly on the bare Brev host (not inside the Docker build) to stage the huggingface-hub download step. The host image does not ship ensurepip, so venv creation fails on every run with:

The virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venv
    apt install python3.12-venv

Seen on PR #1386, run https://github.com/NVIDIA/TensorRT-Model-Connect/actions/runs/35642651683/job/106501782895.

Fix

Install python3-venv via apt on the host before creating the staging venv.

[coderabbitai]

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