ci: add imagemagick

Needed for image conversion on new Ubuntu version.

Author: kruton

.github/workflows/ci.yml

@@ -12,6 +12,8 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+    - uses: mfinelli/setup-imagemagick@v5
+
     - uses: actions/checkout@v2
       with:
         persist-credentials: false