Switch to using setup-action for installing just

Instead of a third-party action.

.github/workflows/test.yml

@@ -10,12 +10,11 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: opensafely-core/setup-action@v1
         with:
           python-version: 3.9
-          cache: "pip"
           cache-dependency-path: requirements.*.txt
-      - uses: extractions/setup-just@v1
+          install-just: true
       - name: Check formatting, linting and import sorting
         run: just check
 
@@ -28,13 +27,11 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: opensafely-core/setup-action@v1
         with:
           python-version: ${{ matrix.python-version }}
-          cache: "pip"
           cache-dependency-path: requirements.*.txt
-
-      - uses: extractions/setup-just@v1
+          install-just: true
       - name: Run tests
         env:
           PYTHON_VERSION: python${{ matrix.python-version }}