Use pre-built docker image for lsif-go

Reasons:
- Building master from scratch is not reproducible
  as repository is being developed
- go get is a way to add dependencies in Go. So if
  the project is written in Go dependency conflict
  might suddenly occur.
See https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/merge_requests/68#note_401431566

doc/user/project/code_intelligence.md

@@ -26,10 +26,9 @@ Enable code intelligence for a project by adding a GitLab CI/CD job to the proje
 
 ```yaml
 code_navigation:
-  image: golang:1.14.0
+  image: sourcegraph/lsif-go:v1
   allow_failure: true # recommended
   script:
-    - go get github.com/sourcegraph/lsif-go/cmd/lsif-go
     - lsif-go
   artifacts:
     reports: