Update .gitlab-ci.yml: try to connect to gitlab, eval ssh agent

.gitlab-ci.yml

-variables:
-  SSH_AUTH_SOCK: "/tmp/ssh-jCGBqSUZJU8o/agent.25566"
-  SSH_AUTH_PID: "25567"
 
 stages:
   - build
@@ -9,8 +6,8 @@ stages:
 build:
   stage: build
   script:
-    - ssh-agent
-    - ssh-add
+    - eval `ssh-agent -s`
+    - ssh-add ~/.ssh/id_rsa.pub
     - ssh -vvv git@gitlab.inria.fr
     - git submodule init
     - git submodule update