ci: update pip via python -m pip

pip install -U pip setuptools wheel leads to an error:
https://github.com/bayesflow-org/bayesflow/actions/runs/14692655483/job/41230057180?pr=449

Author: vpratz

.github/workflows/tests.yaml

@@ -43,7 +43,7 @@ jobs:
 
       - name: Install Dependencies
         run: |
-          pip install -U pip setuptools wheel
+          python -m pip install -U pip setuptools wheel
           pip install .[test]
 
       - name: Install JAX