ci: update minimum coverage requirement

- Adjust minimum coverage from 90% to 88% in GitHub Actions workflow

Author: fulleni

.github/workflows/main.yaml

@@ -16,4 +16,4 @@ jobs:
   build:
     uses: VeryGoodOpenSource/very_good_workflows/.github/workflows/dart_package.yml@v1
     with:
-      min_coverage: 90
+      min_coverage: 88