Update pipeline generation ci file to use sparse checkout

It's not particularly impacting, since this usually runs in the background, but it can be annoying to wait 5+ minutes for repos to check when running it ad-hoc or for PR testing/verification. Plus the [pipeline file](https://github.com/Azure/azure-sdk-tools/blob/main/eng/pipelines/pipeline-generation.yml) could use some cleanup anyway as there is a lot of duplication.