ci: upgrade pip in the venv before installing torch

Debian bookworm's python3-venv seeds pip 23.0.1, which rejects the
PyTorch CPU index's typing_extensions wheel because its metadata spells
the name with an underscore. It then falls back to the sdist, whose build
needs flit_core, which that index does not carry.

.gitea/workflows/tests.yml

@@ -70,6 +70,9 @@ jobs:
           apt-get update
           apt-get install -y --no-install-recommends python3 python3-venv
           python3 -m venv /opt/venv
+          # bookworm's pip 23.0 rejects the PyTorch index's typing_extensions
+          # wheel over its name's spelling, then cannot build it from source
+          /opt/venv/bin/pip install --retries 5 --upgrade pip
           echo /opt/venv/bin >> "$GITHUB_PATH"
 
       # The CPU build first: torch from PyPI is the CUDA build, several GB of