Install zstd in CI test env

.gitlab-ci.yml

@@ -11,7 +11,7 @@ test:
   stage: test
   image: registry.git.autistici.org/ai3/docker/test/golang:master
   script:
-    - apt-get -q update && apt-get -qy install restic lz4
+    - apt-get -q update && apt-get -qy install restic zstd
     - run-go-test ./...
   artifacts:
     when: always