ci: restrict nightly samples jobs to run only in snippets/ (#474)
ci: restrict nightly samples jobs to run only in snippets/ (#474)

To address further RESOURCE_EXHAUSTED errors from nightly samples runs
![image](https://user-images.githubusercontent.com/7338432/97338082-72ab9180-1857-11eb-9bda-dfa1bc28ec11.png)

googleapis/google-cloud-java,googleapis/google-cloud-java,googleapis/google-cloud-java