Install bsdtar in the CI

The recent update to Ubuntu 20.04 from 18.04 resulted in bsdtar being dropped
from the CI environment. This reduces our test coverage, so let's install it
again.

.github/workflows/testing.yml

@@ -41,5 +41,7 @@ jobs:
         with:
           ruby-version: ${{matrix.ruby}}
           bundler-cache: true
+      - name: install dependencies
+        run: sudo apt -y install libarchive-tools
       - name: Run Tests
         run: bundle exec rake test:unit