ci: apt-get update before installing imagemagick (#2780)

update-blog-preview-images has failed on every run since 2026-09-22 18:02,
including on master. The install step runs without apt-get update, so the
runner image's package index goes stale as soon as Ubuntu publishes a
security update, and the pinned ghostscript version 404s:

  E: Failed to fetch .../ghostscript/libgs10_10.02.1~dfsg1-0ubuntu7.8_amd64.deb
     404 Not Found
  E: Unable to fetch some archives, maybe run apt-get update

Unrelated to any content change: it fails during apt, before the script runs.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/generate-preview-images.yml

@@ -17,7 +17,9 @@ jobs:
           fetch-depth: 0
 
       - name: Install cwebp and imagemagick
-        run: sudo apt-get install -y webp imagemagick
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y webp imagemagick
       - name: Run a script
         run: |
           ADD_TO_GIT=true bash -x automation/gen-all-blogs-previews.sh