Switch to modified existing yarn config so it actually works

I hate it, it's nasty, but it actually works. Until https://github.com/mrmartineau/gatsby-theme-code-notes/issues/65 is fixed, it's the best I can do

.gitlab-ci.yml

@@ -3,13 +3,14 @@ image: "node:lts-slim"
 cache:
   key: ${CI_COMMIT_REF_SLUG}
   paths:
-    - .npm
+    - node_modules/
+    - .yarn
     - .cache
 
 pages:
   stage: deploy
   script:
-    - npm ci --cache .npm --prefer-offline --production
+    - yarn install --prefer-offline --production
     - npm run release
   artifacts:
     paths:

package.json

@@ -19,8 +19,7 @@
     "gatsby-theme-code-notes": "^1.15.1",
     "gzipper": "^4.0.2",
     "react": "^16.13.1",
-    "react-dom": "^16.13.1",
-    "theme-ui": "^0.3.1"
+    "react-dom": "^16.13.1"
   },
   "repository": {
     "type": "git",