Avoid generating 3rd party fakes if no change to `vendor/*`

[maximilien]

➜  kn-source-github git:(master) ./hack/build.sh
🚒 Update
🧽 Format
⚖️  License
🎭 Fakes (pkg/types)
🎭 Fakes (3rd parties)
Writing `FakeSourcesV1alpha1Interface` to `pkg/fakes/fake_sources_v1alpha1interface.go`... Done
Writing `FakeGitHubSourcesGetter` to `pkg/fakes/fake_sources_githubsourcesgetter.go`... Done
Writing `FakeGitHubSourceInterface` to `pkg/fakes/fake_sources_githubsourceinterface.go`... Done
...

These 3rd party fakes should be generating when vendor did not change.