Use latest Bundler consistenly in CI

Old Bundler versions will always resolve git gems from the "master"
branch but some gems in our Gemfiles use "main".

Use modern Bundler versions that handle this fine.

.github/workflows/test.yml

@@ -46,6 +46,7 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
+          bundler: latest
 
       - name: Run tests
         run: |