github actions: Install Python dependencies in validate-kernel-commits workflow

The check_kernel_commits.py script requires the gitpython package (imported
as 'git' in ciq_helpers.py), but the workflow was not installing dependencies
after setting up Python. This caused the workflow to fail with:
"ModuleNotFoundError: No module named 'git'"

Add a step to install dependencies from pyproject.toml after Python setup.

Author: bmastbergen

.github/workflows/validate-kernel-commits.yml

@@ -135,6 +135,12 @@ jobs:
         with:
           python-version: '3.x'
 
+      - name: Install Python dependencies
+        working-directory: kernel-src-tree-tools
+        run: |
+          python3 -m pip install --upgrade pip
+          pip install .
+
       - name: Run upstream fixes check
         id: check-kernel-commits
         working-directory: kernel-src-tree-tools