CI: Create empty venv on cache miss
# What does this PR do?

Fixes a CI error introduced by https://github.com/huggingface/transformers/pull/16789 -- when there was a cache miss, it attempted to initialize a virtual environment that didn't exist, in two CI workflows.

This PR fixes that by creating empty venvs on a cache miss.

(I wonder why the original CI run inside the PR didn't fail, the new cache names should have triggered the same error 🤔 )
_The documentation is not available anymore as the PR was closed or merged._