fix: build fail on warnings

Author: fgdorais

.github/workflows/build.yml

@@ -23,6 +23,6 @@ jobs:
         uses: leanprover/lean-action@v1
         with:
           build: true
-          build-args: "UnicodeBasic UnicodeData"
+          build-args: "-wfail UnicodeBasic UnicodeData"
           test: true
           use-mathlib-cache: false