CI: make sure imagemagick is installed

Github actions has now defaulted to using its Ubuntu 24.04 base image.
Compared to the previous base image, this is missing many pre-installed
binaries. Such as imagemagick.

Comparison:

- Ubuntu 24.04: https://github.com/actions/runner-images/blob/ubuntu24/20241215.1/images/ubuntu/Ubuntu2404-Readme.md
- Ubuntu 22.04: https://github.com/actions/runner-images/blob/ubuntu24/20241215.1/images/ubuntu/Ubuntu2204-Readme.md

.github/workflows/test.yml

@@ -34,9 +34,14 @@ jobs:
         python: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     steps:
       - uses: actions/checkout@v4
-      - name: Install optimizers
+      - name: Install required system packages
         run: |
-          sudo apt-get install -y jpegoptim pngquant gifsicle optipng libjpeg-progs webp
+          sudo apt-get update -y
+          sudo apt-get install -y \
+            # For Wand tests
+            imagemagick \
+            # For testing optimizers
+            jpegoptim pngquant gifsicle optipng libjpeg-progs webp
       - name: Set up Python ${{ matrix.python }}
         uses: actions/setup-python@v5
         with: