Can't run `go generate` if rust is not installed

Created by: jhchabran

Due to https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+dev/proto-generate.sh&patternType=literal, we can't run go generate if rust is not installed.

Either we need to separate this generation target or we need to add rust to sg setup.