Use pipx for installing Poetry (#51)

[pipx is a pre-installed package
manager](https://github.com/actions/runner-images/blob/266f9413d39fc77ade974757b633ef98873c9c21/images/ubuntu/Ubuntu2204-Readme.md#package-management)
in the latest Ubuntu GHA runner images.
Since it is a recommended way of installing Poetry, because it creates a
separate environment, I changed the workflows to install Poetry through
pipx.
In order to leverage `poetry.lock`-dependent [caching
feature](https://github.com/actions/setup-python/blob/e9d6f990972a57673cdb72ec29e19d42ba28880f/docs/advanced-usage.md#caching-packages)
offered by
[actions/setup-python](https://github.com/actions/setup-python), I moved
the Poetry setups before the `setup-python` step is performed.
Additionally, I bumped actions/setup-python to the latest v5 version
(see
[release](https://github.com/actions/setup-python/releases/tag/v5.0.0)
for details on what's changed).

.github/workflows/check.yaml

@@ -29,14 +29,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Set up Poetry
+        run: |
+          pipx install poetry==${{ env.POETRY_VERSION }}
+
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v5
         with:
           python-version: "${{ matrix.python-version }}"
-
-      - name: Set up Poetry
-        run: |
-          pip install poetry==${{ env.POETRY_VERSION }}
+          cache: poetry
 
       - name: Install packages
         run: |

.github/workflows/publish.yaml

@@ -35,14 +35,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Set up Poetry
+        run: |
+          pipx install poetry==${{ env.POETRY_VERSION }}
+
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
-
-      - name: Set up Poetry
-        run: |
-          pip install poetry==${{ env.POETRY_VERSION }}
+          cache: poetry
 
       - name: Install zsh
         run: |

.github/workflows/test.yaml

@@ -41,14 +41,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Set up Poetry
+        run: |
+          pipx install poetry==${{ env.POETRY_VERSION }}
+
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
-
-      - name: Set up Poetry
-        run: |
-          pip install poetry==${{ env.POETRY_VERSION }}
+          cache: poetry
 
       - name: Install packages
         run: |