Fix CI/CD fail by installing ImageMagick on runner

This commit addresses an issue where CI/CD jobs fail due to the removal of
`imagemagick` from GitHub's preinstalled software list for Ubuntu
runners. As a result, commands relying on `imagemagick` were not found.

To resolve this, the commit installs `imagemagick` across all platforms
(Linux, macOS, and Windows). This safeguards against future changes to
the preinstalled software list on GitHub runners.

This commit also centralizes installing of ImageMagick as its own action
for reusability.

.github/workflows/checks.desktop-runtime-errors.yaml

@@ -26,6 +26,9 @@ jobs:
       -
         name: Install dependencies
         uses: ./.github/actions/npm-install-dependencies
+      -
+        name: Install ImageMagick # For screenshots
+        uses: ./.github/actions/install-imagemagick
       -
         name: Configure Ubuntu
         if: contains(matrix.os, 'ubuntu') # macOS runner is missing Docker
@@ -56,9 +59,6 @@ jobs:
           sudo Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
           echo "DISPLAY=:99" >> $GITHUB_ENV
 
-          # Install ImageMagick for screenshots
-          sudo apt install -y imagemagick
-
           # Install xdotool and xprop (from x11-utils) for window title capturing
           sudo apt install -y xdotool x11-utils
       -