[rubygems/rubygems] Consistently assert command failure

https://github.com/rubygems/rubygems/commit/faaa594a4a

spec/bundler/cache/gems_spec.rb

@@ -167,7 +167,7 @@ RSpec.describe "bundle cache" do
         G
 
         bundle :cache, raise_on_error: false
-        expect(exitstatus).to_not eq(0)
+        expect(last_command).to be_failure
         expect(err).to include("json-#{default_json_version} is built in to Ruby, and can't be cached")
       end
     end

spec/bundler/install/gemfile/git_spec.rb

@@ -1068,7 +1068,7 @@ RSpec.describe "bundle install with git sources" do
       gem "foo", :git => "#{lib_path("foo-1.0")}"
     G
 
-    expect(exitstatus).to_not eq(0)
+    expect(last_command).to be_failure
     expect(err).to include("Bundler could not install a gem because it " \
                            "needs to create a directory, but a file exists " \
                            "- #{default_bundle_path("bundler")}")