test: add CI test that vercel.json does not change (#1986)

* test: add CI test that vercel.json does not change

* fix: fixup

* test: debug

* test: do not use tmp file

.circleci/config.yml

@@ -70,9 +70,20 @@ jobs:
               sleep 1
             done
             echo Failed waiting for redis && exit 1
+      - run:
+          name: Copy vercel.json
+          command: cp vercel.json vercel-old.json
       - run:
           name: Build
           command: yarn build
+      - run:
+          name: Check vercel.json unchanged
+          command: |
+            if ! diff -q vercel-old.json vercel.json &>/dev/null; then
+              diff vercel-old.json vercel.json
+              echo "vercel.json changed, run yarn build and make sure everything looks okay"
+              exit 1
+            fi
       - run:
           name: Integration tests
           command: yarn test-in-ci