Move website build to a composite action (so it can be reused)

.github/actions/build-website/action.yml

@@ -0,0 +1,20 @@
+name: Build website
+runs:
+  using: composite
+  steps:
+    - name: Checkout repo
+      uses: actions/checkout@v4
+
+    - name: Set up Python 3.9
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.9'
+        cache: 'pip'
+
+    - name: Install dependencies
+      run: |
+        pip install -r requirements.txt
+
+    - name: Build website
+      run: |
+        nikola build

.github/workflows/publish.yml

@@ -9,22 +9,8 @@ jobs:
     runs-on: ubuntu-22.04
 
     steps:
-      - name: Checkout repo
-        uses: actions/checkout@v4
-
-      - name: Set up Python 3.9
-        uses: actions/setup-python@v5
-        with:
-          python-version: '3.9'
-          cache: 'pip'
-
-      - name: Install dependencies
-        run: |
-          pip install -r requirements.txt
-
       - name: Build website
-        run: |
-          nikola build
+        uses: ./.github/actions/build-website
 
       - name: Install SSH key
         uses: shimataro/ssh-key-action@v2