ci: refactor pipenv installation steps across all workflow files

Author: DhavalGojiya

.github/workflows/documentation.yml

@@ -37,8 +37,7 @@ jobs:
         with:
           python-version: "3.10"
       - name: Install pipenv
-        run: |
-          pip install pipenv
+        run: pip install pipenv
       - name: Install dependencies
         run: |
           pipenv --python $(which python) install --dev

.github/workflows/tests.yml

@@ -45,7 +45,7 @@ jobs:
           python-version: "3.9"
           cache: "pipenv"
       - name: Install pipenv
-        run: pipx install pipenv
+        run: pip install pipenv
       - name: Install dependencies
         run: pipenv install --dev --python=${{ matrix.python-version }}
       - name: Linter with pylint