discovery: add GCP service account JSON file to .gitignore
discovery: add GCP service account JSON file to .gitignore

Change-Id: I8d7fa5e13d67af93e5ab9208ad9084598bb8a091
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/441335
Reviewed-by: Channing Kimble-Brown <553f30b1a1aaa6929be75c10f5dad40052105e72@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite