fix github workflow pytest

Author: berndverst

.github/workflows/pr-validation.yml

@@ -25,11 +25,12 @@ jobs:
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
-    - name: Install durabletask dependencies
+    - name: Install durabletask dependencies and the library itself in editable mode
       run: |
         python -m pip install --upgrade pip
         pip install flake8 pytest
         pip install -r requirements.txt
+        pip install -e .
     - name: Install durabletask-azuremanaged dependencies
       working-directory: examples/dts
       run: |