Duplicate frozen CHILD_STATUS to allow rspec proxy

Without this, a test fails because rspec is unable to proxy $CHILD_STATUS/$?. I looked around for another way to handle this but there's no way to unfreeze except through duplication.

```
1) Bundle when the system call fails emits all stdout output even if verbose is false
     Failure/Error: success = st.success?

     ArgumentError:
       Cannot proxy frozen objects, rspec-mocks relies on proxies for method stubbing and expectations.
     # ./lib/bundle/bundle.rb:18:in `block in system'
     # ./lib/bundle/bundle.rb:12:in `popen'
     # ./lib/bundle/bundle.rb:12:in `system'
     # ./spec/bundle_utils_spec.rb:22:in `block (4 levels) in <top (required)>'
     # ./spec/bundle_utils_spec.rb:22:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:73:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:73:in `block (2 levels) in <top (required)>'
     # ------------------
     # --- Caused by: ---
     # FrozenError:
     #   can't modify frozen object: pid 0 exit 0
     #   ./lib/bundle/bundle.rb:18:in `block in system'
```

lib/bundle/bundle.rb

@@ -14,7 +14,7 @@ def system(cmd, *args, verbose: false)
           logs << buf
         end
         Process.wait(pipe.pid)
-        success = $CHILD_STATUS.success?
+        success = $CHILD_STATUS.dup.success?
         pipe.close
       end
       puts logs.join unless success