Bump cache action from v2 to v4 (#14)

Author: phillipuniverse

.github/workflows/build.yml

@@ -28,7 +28,7 @@ jobs:
           virtualenvs-in-project: true
       - name: Load cached venv
         id: cached-poetry-dependencies
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: .venv
           key: venv-${{ runner.os }}-${{ matrix.python-version }}-${{ hashFiles('**/poetry.lock') }}