CI: use venv for Python tests to isolate setuptools from system Python

deadsnakes python3.12 sees system python3.10's setuptools at
/usr/lib/python3/dist-packages and skips installation, leaving python3.12
without setuptools. A venv gets its own site-packages so pip/setuptools
are properly present for http-ece's legacy setup.py build.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -86,16 +86,17 @@ jobs:
           apt-get install -y -qq python3.12 python3.12-dev python3.12-venv python3-pip git
           python3.12 -m ensurepip --upgrade
 
-      - name: Install setuptools and pre-build legacy deps
-        run: |
-          python3.12 -m pip install setuptools wheel
-          python3.12 -m pip install --no-build-isolation http-ece
+      - name: Create virtual environment
+        run: python3.12 -m venv /opt/venv
 
       - name: Install package with dev deps
-        run: python3.12 -m pip install -e ".[dev]"
+        run: |
+          /opt/venv/bin/pip install --upgrade pip setuptools wheel
+          /opt/venv/bin/pip install --no-build-isolation http-ece
+          /opt/venv/bin/pip install -e ".[dev]"
 
       - name: Run tests
-        run: python3.12 -m pytest tests/ -v
+        run: /opt/venv/bin/python -m pytest tests/ -v
 
   build:
     name: Build & push image