setup-macos: Re-enable installation of imagemagick

Bug: #3172

Author: wantehchang

.github/actions/setup-macos/action.yml

@@ -60,9 +60,7 @@ runs:
     - name: Install imagemagick
       # imagemagick is used in sh tests (not gtest tests) but we assume that no tests are run by the CI
       # if gtest is off
-      # TODO: issue #3172 - Disable temporarily.
-      # if: ${{ inputs.gtest != 'OFF' }}
-      if: false
+      if: ${{ inputs.gtest != 'OFF' }}
       run: echo "AVIF_MAC_LIBRARIES=${{ env.AVIF_MAC_LIBRARIES }} imagemagick" >> "$GITHUB_ENV"
       shell: bash
     - name: Install gtest