CI: remove redundant DEBIAN_FRONTEND=noninteractive env

The Linux runners on GitHub Actions already sets this env variable,
see:
https://github.com/actions/runner-images/blob/2a4bc14da46f1f8e358aa902a69edb9bef135472/images/ubuntu/scripts/build/configure-dpkg.sh#L17

Author: kleisauke

.github/workflows/test.yml

@@ -56,8 +56,6 @@ jobs:
           bundler-cache: true
 
       - name: Install libvips
-        env:
-          DEBIAN_FRONTEND: noninteractive
         run: |
           sudo apt-get update
           # we only need the library