fix(ci): install imagemagick in runner

The latest runner image, ubuntu-24.04 does not have imagemagick
installed. (The previous image, ubuntu-22.04, did.)

.github/workflows/deploy.yml

@@ -39,6 +39,11 @@ jobs:
           python -m pip install --upgrade pip setuptools
           python -m pip install lektor
 
+      - name: Install Imagemagick
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y imagemagick
+
       - name: Build lektor website
         run: lektor build -f webpack -O '${{ env.OUTPUT }}'