github workflow: use python3 consistently

.github/workflows/test.yml

@@ -17,7 +17,7 @@ jobs:
         python-version: '3.12'
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
+        python3 -m pip install --upgrade pip
         pip install .
     - name: Run tests
-      run: python -m exo.inference.test_inference_engine
+      run: python3 -m exo.inference.test_inference_engine