re-add pip upgrade

Author: mhubii

.github/workflows/tests.yaml

@@ -9,6 +9,8 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
+    env:
+      PIP_NO_CACHE_DIR: "1"
 
     strategy:
       matrix:
@@ -54,6 +56,7 @@ jobs:
       # Install roboreg (and its dependencies from pyproject.toml).
       - name: Install package and dependencies
         run: |
+          python -m pip install --upgrade pip --no-cache-dir
           pip install . --no-build-isolation --no-cache-dir
 
       # Run tests with pytest.