add code coverage gate to CI

.forgejo/workflows/build.yaml

@@ -20,6 +20,8 @@ jobs:
         run: zig build --summary all
       - name: Run tests
         run: zig build test -Ddownload-geoip --summary all
+      - name: Check code coverage >= 80%
+        run: zig build coverage -Ddownload-geoip -Dcoverage-threshold=80 --summary all
       - name: Package
         run: zig build -Dtarget="$BUILD_TARGET" -Doptimize="$BUILD_OPTIMIZATION"
       - name: Upload