Try using ImageMagick binary from website

Alternative to #375, try using the ImageMagick release directly from the
website. It may have additional capabilities compared to the GitHub
release (like HEIC support, maybe).

.github/workflows/ci.yaml

@@ -14,7 +14,6 @@ env:
   # Paths for various manually installed utilities. Don't try to use $HOME
   # because GitHub Actions won't support it.
   MAGICK_BIN: /home/runner/imagemagick/bin/magick
-  MAGICK_RELEASE: 7.1.1-18
   MOZJPEG_BIN: /opt/mozjpeg/bin/cjpeg
   PNGQUANT_BIN: /usr/bin/pngquant
 
@@ -59,7 +58,7 @@ jobs:
       - 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
+          curl --compressed -L -o "${{ env.MAGICK_BIN }}" https://imagemagick.org/archive/binaries/magick
           chmod +x "${{ env.MAGICK_BIN }}"
 
       - name: ImageMagick format options
@@ -160,7 +159,7 @@ jobs:
       - 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
+          curl --compressed -L -o "${{ env.MAGICK_BIN }}" https://imagemagick.org/archive/binaries/magick
           chmod +x "${{ env.MAGICK_BIN }}"
 
       # See notes in check_and_test.
@@ -222,7 +221,7 @@ jobs:
       - 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
+          curl --compressed -L -o "${{ env.MAGICK_BIN }}" https://imagemagick.org/archive/binaries/magick
           chmod +x "${{ env.MAGICK_BIN }}"
 
       # See notes in check_and_test.