Cache go build artifacts (#619)

.circleci/config.yml

@@ -99,10 +99,17 @@ jobs:
           name: Watch nodes
           command: kubectl get nodes -o json --watch
           background: true
+      - restore_cache:
+          keys:
+            - v1-pkg-cache
       - run:
           name: Run the end-to-end test suite
           command: |
             make e2e
+      - save_cache:
+          key: v1-pkg-cache
+          paths:
+            - "/go/pkg"
       - run:
           name: Collect logs on failure from vkubelet-mock-0
           command: |