ci: try to use sccache

.gitlab-ci.yml

@@ -42,8 +42,15 @@ tests:
     - if: '$CI_MERGE_REQUEST_ID ||$CI_COMMIT_BRANCH == "master" || $CI_COMMIT_BRANCH =~ /^release/'
     - when: manual
   stage: tests
+  cache:
+    - key:
+        files:
+          - Cargo.lock
+      paths:
+        - .target
   variables:
     DUNITER_INTEGRATION_TESTS_SPAWN_NODE_DURATION: "10"
+    RUSTC_WRAPPER: "/root/.cargo/bin/sccache"
   script:
     - cargo build
     - cargo test