Remove createBuilder Func from build_kubectl_buildkit_test.go if Upstream Issue is Resolved

Opening this issue to give more visibility around eventually removing [`createBuilder` in `build_kubectl_buildkit_test.go`](https://github.com/vmware-tanzu/carvel-kbld/blob/1997580f07463f98502a557649377f79cb594380/test/e2e/build_kubectl_buildkit_test.go#L119) that was added in #73.

If the buildkit issue [here](https://github.com/vmware-tanzu/buildkit-cli-for-kubectl/issues/55) is resolved, the function should eventually be removed. As part of closing this issue, it should be confirmed that tests are not flaky as documented in #69 and that tests run successfully without needing to create the builder.

More information on the need for the function is documented [here](https://github.com/vmware-tanzu/carvel-kbld/blob/1997580f07463f98502a557649377f79cb594380/test/e2e/build_kubectl_buildkit_test.go#L112).