ci(gitlab): update npm

- partially revert 1610386252

.gitlab-ci.yml

@@ -7,10 +7,11 @@ cache: # add cache to 'node_modules' for speeding up builds
 test:
   before_script:
     - 'which ssh-agent || (apk update && apk add openssh-client git)'
+    - npm install -g npm
     - npm install --only=prod
 
   script:
-    - npm install --only=prod snyk && npm run snyk # Refer to "scripts" in package.json
+    - npm install snyk && npm run snyk # Refer to "scripts" in package.json
     - npm run hexo
 
   # Update github mirror