CONTRIBUTING.md: avoid recommendation to use 'go get'

Since #25325 landed, 'go get' no longer works. Adjust our contributing
guidelines appropriately.

Release note: None

CONTRIBUTING.md

@@ -50,7 +50,7 @@ new engineers.
 2. Get the CockroachDB code:
 
    ```shell
-   go get -d github.com/cockroachdb/cockroach
+   git clone https://github.com/cockroachdb/cockroach $(go env GOPATH)/src/github.com/cockroachdb/cockroach
    cd $(go env GOPATH)/src/github.com/cockroachdb/cockroach
    ```