Try bundled ImageMagick again

Now that Ubuntu's upgraded to 24.04 on GitHub Actions, maybe there's an
ImageMagick that's been built sometime in the last century, and my
custom build is no longer working anyway.

.github/workflows/ci.yaml

@@ -49,18 +49,8 @@ jobs:
       #   * WebPs are run through `cbwebp` (which comes from the webp package)
       #     by ImageMagick.
       #
-      - name: Install PNGQuant + WebP
-        run: sudo apt-get install pngquant webp
-
-      # The copy of ImageMagick we could get from apt-get is ancient and
-      # doesn't handle niceties like `.heic` files, so here we get a binary
-      # directly. When Ubuntu is upgraded at some point in the probably-distant
-      # future, we can probably get rid of this.
-      - name: Install ImageMagick
-        run: |
-          mkdir -p $(dirname "${{ env.MAGICK_BIN }}")
-          curl --compressed -L -o "${{ env.MAGICK_BIN }}" https://github.com/brandur/imagemagick-builder/releases/download/${{ env.MAGICK_RELEASE }}/magick
-          chmod +x "${{ env.MAGICK_BIN }}"
+      - name: Install ImageMagick + PNGQuant + WebP
+        run: sudo apt-get install imagemagick pngquant webp
 
       - name: ImageMagick format options
         run: ${{ env.MAGICK_BIN }} identify -list format