ci: fix GitHub Actions tests by removing invalid uv source command

`uv` doesn’t support `source`. Virtualenv activation isn’t needed in CI since
`uv run` handles execution inside the `.venv`. This fixes the failing workflow.

Author: theolujay

.github/workflows/tests.yml

@@ -25,10 +25,8 @@ jobs:
 
             - name: Install dependencies
               run: |
-                python -m pip install --upgrade pip
                 pip install uv
                 uv venv
-                uv source .venv/bin/activate
                 uv pip install -e ".[dev]"
                 uv pip install pytest pytest-cov