Install the shared Molecule requirements in CI too

The previous commit moved requirements.txt into molecule-shared/ but left
the workflow installing from the old per-role path, so CI failed on a file
that no longer exists while the local run - which reads the shared file
through bin/molecule.sh - passed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/molecule.yml

@@ -143,7 +143,7 @@ jobs:
           python-version: "3.x"
 
       - name: Install test dependencies
-        run: python3 -m pip install -r roles/custom/${{ matrix.role }}/molecule/requirements.txt
+        run: python3 -m pip install -r molecule-shared/requirements.txt
 
       - name: Run Molecule
         working-directory: roles/custom/${{ matrix.role }}