ci: Convert jobs to test with bundled Git only by default

Currently most of our tests are set up with both a bundled Git and Git distribution installed. This made sense back when we still had the feature flag that enabled us to switch between both versions, but that flag is gone by now. This MR thus converts our CI jobs to only have either bundled Git installed (which now is the default configuration) or instead to have a Git distribution installed (which we only do for the minimum required Git version).