Run Pytest in installed package

This avoids the `import file mismatch` error from Pytest.

.github/workflows/python-package.yml

@@ -118,14 +118,20 @@ jobs:
       - name: Run Pytest with the Darker plugin on recent Black versions
         if: matrix.constraints != '--constraint constraints-oldest.txt'
         run: |
-          pytest --darker
+          pytest --darker $(python -c "
+          import os, darker
+          print(os.path.dirname(darker.__file__))
+          ")
       - name: Run Pytest without the Darker plugin on oldest Black version
         # The reformatting rules used to be a bit different. We don't need to
         # test reformatting Darker's own code base with old Black versions.
         # Interoperability is ensured by unit tests.
         if: matrix.constraints == '--constraint constraints-oldest.txt'
         run: |
-          pytest
+          pytest $(python -c "
+          import os, darker
+          print(os.path.dirname(darker.__file__))
+          ")
 
   build-sdist-validate-dists:
     runs-on: ubuntu-latest