CI: Install manim system dependencies on Ubuntu

Manim requires pango, cairo, and ffmpeg on Linux. These are bundled on
macOS and Windows but need apt-get installation on Ubuntu runners.

Author: jeremymanning

.github/workflows/ci.yml

@@ -19,6 +19,12 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Install system dependencies (Ubuntu)
+        if: runner.os == 'Linux'
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libpango1.0-dev libcairo2-dev ffmpeg
+
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v5
         with:
@@ -56,6 +62,11 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Install system dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libpango1.0-dev libcairo2-dev ffmpeg
+
       - name: Set up Python
         uses: actions/setup-python@v5
         with: