ci: retry checkouts since they sporadically fail

NO_DOC=internal
NO_TEST=internal
NO_CHANGELOG=internal

.gitlab-ci.yml

@@ -4,9 +4,11 @@ variables:
     USER: 'tarantool'
     DOCKER_REGISTRY_PUB: docker-public.binary.picodata.io
     DOCKER_AUTH_CONFIG: $DOCKER_AUTH_RW
-  # k8s runner config
+    # k8s runner config
     KUBERNETES_CPU_REQUEST: 4
     KUBERNETES_MEMORY_REQUEST: "8Gi"
+    # Helps to tolerate spurious network failures
+    GET_SOURCES_ATTEMPTS: 3
 
 stages:
   - test