Revert "ci(gitlab): switch npm to pnpm"

This reverts commit ff62fb9ad4.

.gitlab-ci.yml

@@ -7,12 +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 pnpm
-    - pnpm install
+    - npm install --only=prod
 
   script:
-    - pnpm install snyk && pnpm run snyk # Refer to "scripts" in package.json
-    - pnpm run hexo
+    - npm install --only=prod snyk && npm run snyk # Refer to "scripts" in package.json
+    - npm run hexo
 
   # Update github mirror
   after_script: